Noble metal–metal oxide nanohybrids with tailored ...nanofm.mse.gatech.edu/Papers/X. Liu et al. EES. 2017, 10, 402.pdfNoble metal–metal oxide nanohybrids with tailored nanostructures

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REVIEW ARTICLEZhen Li, Zhiqun Lin et al.Noble metal–metal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion, photocatalysis and environmental remediation

Volume 10 Number 2 February 2017 Pages 385–644

402 | Energy Environ. Sci., 2017, 10, 402--434 This journal is©The Royal Society of Chemistry 2017

Cite this: Energy Environ. Sci.,

2017, 10, 402

Noble metal–metal oxide nanohybrids withtailored nanostructures for efficient solar energyconversion, photocatalysis and environmentalremediation

Xueqin Liu,ab James Iocozzia,b Yang Wang,a Xun Cui,b Yihuang Chen,b

Shiqiang Zhao,b Zhen Li*a and Zhiqun Lin*b

The controlled synthesis of nanohybrids composed of noble metals (Au, Ag, Pt and Pd, as well as AuAg

alloy) and metal oxides (ZnO, TiO2, Cu2O and CeO2) have received considerable attention for applications in

photocatalysis, solar cells, drug delivery, surface enhanced Raman spectroscopy and many other important

areas. The overall architecture of nanocomposites is one of the most important factors dictating the physical

properties of nanohybrids. Noble metals can be coupled to metal oxides to yield diversified nanostructures,

including noble metal decorated-metal oxide nanoparticles (NPs), nanoarrays, noble metal/metal oxide core/

shell, noble metal/metal oxide yolk/shell and Janus noble metal–metal oxide nanostructures. In this review,

we focus on the significant advances in tailored nanostructures of noble metal–metal oxide nanohybrids.

The improvement in performance in the representative solar energy conversion applications including

photocatalytic degradation of organic pollutants, photocatalytic hydrogen generation, photocatalytic CO2

reduction, dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are discussed. Finally, we con-

clude with a perspective on the future direction and prospects of these controllable nanohybrid materials.

Broader contextRapid population growth and industrialization have produced concerns regarding energy availability and environmental pollution. Addressing these problemshas been the focus of research the world over. Solar energy, a clean and effectively infinite energy source, can be efficiently harnessed by photocatalytic andphotovoltaic processes to aid in environmental remediation and green energy generation. Hybrid nanomaterials composed of metal oxides and noble metalsare promising candidates for such purposes because of their enhanced light harvesting ability and high degree of charge carrier separation. In this review, wefocus on the preparation and properties of noble metal–metal oxide nanocomposites with several different nanostructures including noble metal-decoratedmetal oxide nanoparticles and nanoarrays, noble metal/metal oxide core/shell and yolk/shell nanostructures, and Janus noble metal–metal oxidenanostructures. The applications of these materials related to solar energy conversion in photocatalytic degradation of environmental contaminants, watersplitting into H2, photocatalytic reduction of CO2, dye-sensitized solar cells, and perovskite solar cells are provided. Lastly, the challenges and future directionsof these materials are highlighted. We hope this review will serve as a useful guide in the design of noble metal–metal oxide hybrid nanostructures for solarenergy applications for both experts and novices alike.

1. Introduction

Fossil fuels, including coal, oil and natural gas, are the corner-stone of our modern civilization.1,2 However, they are beingrapidly consumed with the growth of industry and populationleading to various environmental concerns.3–5 Solar energy is asafe, abundant, green, and effective alternative energy source.6–9

Consequently, figuring out how to efficiently transfer solarenergy into usable energy is a significant challenge.10 Photo-catalytic and photovoltaic processes are accepted as desirableways to convert solar energy efficiently. Under the irradiation ofsunlight, photocatalysts can split water into hydrogen and oxygengas, convert CO2 into fuels and chemicals, and degrade organicpollutants into H2O and CO2 without introducing additionalpollutants.11–13 Photovoltaic devices can directly convert solarenergy into electricity. Recently, perovskite solar cells havebecome one of the most popular photovoltaic device types.14–16

Hybrid nanomaterials combining optical, electronic andmagnetic properties have attracted much interest in recent

a Faculty of Materials Science and Chemistry, China University of Geosciences,

Wuhan, Hubei 430074, China. E-mail: [email protected] School of Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, USA. E-mail: [email protected]

Received 4th August 2016,Accepted 17th August 2016

DOI: 10.1039/c6ee02265k

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years due to their wide-ranging applications in environmentremediation and solar energy conversion.17–20 In general, thesuccessful application of such nanohybrids is determinedby their structure, composition, particle size and otherparameters.21,22 One of the major challenges in this area isthe integration of two dissimilar materials with differentstructures to yield unique hybrid systems with diverse func-tionality. So far, several varieties of nanohybrids have beendeveloped.23–28 Among the different kinds of nanomaterials,noble metal–metal oxide nanocomposites (NMMOs) withtailored structures have developed into an important researcharea due to their attractive photocatalytic29–31 and photo-voltaic32–34 properties.

Metal oxide nanoparticles (MOs), one of the most impor-tant photocatalytic and photovoltaic materials, have beenheavily investigated. However as single-component nano-structures, pure MOs usually exhibit weak absorption of

visible light. For example, TiO2 can only absorb light witha wavelength less than 387 nm.35–37 Various efforts haveattempted to improve the light harvesting of MOs includingdoping,38,39 dye sensitization40–42 and modification withnoble metal NPs (NMs).43–46 A critical part of these methods,the controlled incorporation of MOs with NMs such asAu and Ag, is an effective way to produce nanomaterialsthat are responsive to visible light with prolonged photo-generated charge carrier lifetimes. Since ancient times,NMs have been used for religious purposes and artisticapplications because of their fascinating colors.22,47 Inrecent years, NMs have been found to play important rolesin a variety of applications, such as catalysis andphotovoltaics.48,49 When MOs are coupled with NMs, the lightabsorption range of wide band gap MOs can be extended.50 Inaddition, the formation of Schottky barriers significantlyreduces the recombination of photo-excited electron–holes.51

Xueqin Liu

Xueqin Liu is an AssociateProfessor in the Faculty ofMaterials Science and Chemistryat the China University of Geo-sciences (Wuhan). He received hisPhD in Materials Science andEngineering from the ChinaUniversity of Geosciences (Wuhan)in 2016. He has spent two years inProf. Zhiqun Lin’s group at theGeorgia Institute of Technology asa visiting PhD student since 2014.His research interests includethe preparation of metal oxide

nanoarrays-based nanocomposites and their applications in solarenergy conversion and environment remediation.

James Iocozzia

James Iocozzia is a graduatestudent in the School of MaterialsScience and Engineering at theGeorgia Institute of Technology.He received his Bachelor ofScience in Polymer and FiberEngineering from the GeorgiaInstitute of Technology in 2012.His research interests includenanocomposites, block copolymersand hyperbranched polymersystems for the development offunctional hard and soft organic/inorganic nanomaterials in the

areas of drug delivery, electronics, energy and stimuli-responsivematerials. He is a National Defense Science and EngineeringGraduate (NDSEG) Fellow, a Graduate Student Presidential Fellow,an NSF EAPSI Fellow and a BIONIC Scholar.

Yang Wang

Yang Wang received his BSc in theCollege of Mechanical and Elec-tronic Engineering at the ChinaUniversity of Petroleum, Qingdao,China in 2012. Currently he ispursuing his PhD degree under thesupervision of Prof. Zhen Li in theFaculty of Material Science andChemistry, China University ofGeosciences, Wuhan, China. Hiscurrent research is focused on thephotoelectrochemical properties ofgraphene–TiO2 hybrid semiconduc-tors and efficient electron transfermaterials in photoelectrochemicalcells.

Xun Cui

Xun Cui is a PhD student in theCollege of Chemistry and ChemicalEngineering at the Chongqing Uni-versity. He received his Bachelorof Engineering in ChemicalEngineering and Technology fromJianghan University in 2012. He iscurrently a visiting PhD student inProf. Zhiqun Lin’s group at theGeorgia Institute of Technology.His research interests focus on thedevelopment of advanced materialsfor fuel cells, electrocatalytic andphotocatalytic water splitting, andperovskite solar cells.

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Consequently, NM–MOs are ideal candidates for solar energy-related materials.52

According to the geometrical configuration of nanohybrids,NM–MOs can be divided into five categories: (1) noble metal-decorated metal oxide NPs; (2) noble metal-decorated metaloxide nanoarrays; (3) noble metal/metal oxide core/shell nano-structures; (4) noble metal/metal oxide yolk/shell nanostruc-tures; and (5) Janus noble metal–metal oxide nanostructures(Fig. 1). A number of review articles have been published on thesynthesis, composition, properties and applications of NM–MOs.For instance, Tang et al. focused on the preparation of noblemetal/metal oxide core (yolk)/shell nanostructures and their appli-cations in catalysts.53 Wang et al. have reviewed hybrid nano-structures composed of NMs and MOs. Plasmonic applications

have also been demonstrated.54 However, none of the previousreviews concentrated on the dependence of preparation, propertiesand applications of NM–MOs on the different nanostructure typesavailable.

In this review, we concentrate on recent developments ofdifferent nanostructures of noble metal–metal oxide nano-hybrids including their preparation, properties and perfor-mance related to solar energy conversion in photocatalysisand photovoltaic cells. First, the preparative methods andstructural characteristics of five different NM–MO nanostruc-tures are addressed. Next, the enhanced photocatalytic andphotovoltaic performance of these different nanostructuresare demonstrated by comparison to pure MOs. Finally, weconclude with a perspective on the emerging challenges and

Fig. 1 Schematic representation of different structures of noble metal–metal oxide nanocomposites summarized in this review.

Zhen Li

Zhen Li is a Professor in the Facultyof Materials Science and Chemistry,China University of Geosciences(Wuhan). She received her PhDin mineral-petrological materialsscience from China University ofGeosciences (Wuhan) in 2004. Herresearch is devoted to the develop-ment and application of graphitematerials, the growth andapplication of multifunctionalnanocrystals, the comprehensiveutilization of non-metallicminerals and the preparation offunctional nanocomposites.

Zhiqun Lin

Zhiqun Lin is a Professor in theSchool of Materials Science andEngineering at the GeorgiaInstitute of Technology. Hereceived his PhD in PolymerScience and Engineering from theUniversity of Massachusetts,Amherst in 2002. His researchinterests include perovskite solarcells, polymer solar cells, dye-sensitized solar cells, photo-catalysis, hydrogen generation,lithium ion batteries, semi-conductor organic–inorganic nano-

composites, quantum dots (rods), conjugated polymers, block copolymers,polymer blends, hierarchical structure formation and assembly, surfaceand interfacial properties, multifunctional nanocrystals, and Janusnanostructures.

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future developments of various nanohybrids with differentnanostructures.

2. Structure, properties and synthesisof noble metal–metal oxidenanocomposites (NM–MOs)

In addition to the size and shape, the overall architecture is alsoa key factor in determining the properties of NM–MOs. Theability to controllably integrate NMs with MOs is of specialinterest because it can allow us to understand the interactionbetween NMs and MOs to obtain metal–semiconductor hetero-structures with desired properties. Based on the method ofcombination of NMs and MOs, the resulting NM–MO nano-structure can be divided into three categories: (i) metal oxideinner core and noble metal surface deposition, such as noblemetal-decorated metal oxide NPs/nanoarray nanostructures(Fig. 2a and b);55,56 (ii) noble metal inner core fully coveredby a metal oxide shell, such as NMs/MOs core/shell and yolk/shell nanostructures, as shown in Fig. 2c and d;57,58 (iii) fusednoble metal and metal oxide structures, such as Janus noblemetal–metal oxide nanostructures, as shown in Fig. 2e.59 Thesenanostructures have two advantages compared to bare MOs.One is the extension of the responsive wavelength region ofMOs into the visible region due to the plasmonic effect ofNMs.60,61 Second is a reduction of photogenerated electron–hole recombination because of the formation of Schottkybarriers between NMs and MOs which drives the separationof photo-generated electrons and holes. In addition to thesetwo merits, each nanostructure has unique characteristics. Inthis section we summarize the pros, cons, and fabricationtechniques of different nanostructures of NM–MOs.

2.1 Noble metal-decorated metal oxide nanoparticles

2.1.1 Preparation. The preparation process is the basis forthe properties and applications of nanocomposites. To realizenoble metal-decorated metal oxide nanoparticles (NPs), variousmethods have been developed of which a representative samplingwill be discussed in the following section.

Chemical reduction (CR)62 is one of the simplest methods tointegrate NMs onto the surface of MOs. Typically, this strategyconsists of the immersion of the pre-formed MOs, which act astemplates for the nucleation and growth of NMs, into a noblemetal precursor solution. The precursors then adsorb on thesurface of the MOs followed by reduction through an appro-priate chemical reducing agent. It has been shown that thechoice of reducing agent plays an important role in thismethod. Various reducers, such as sodium borohydride,sodium citrate, ascorbic acid and others agents, have beenemployed.62–65 TiO2/Au nanocomposites have been preparedusing ascorbic acid reducer with monodisperse colloidal TiO2

nanospheres acting as templates via a facile sol–gel method.The composition of the TiO2/Au materials and the size of thedeposited Au NPs are easily controlled by performing sequentialreduction steps as shown in Fig. 3a.63 The preparation proceduredeveloped could also be extended to the synthesis of Ag, Pd andPt-decorated TiO2 composite NPs. During the process of noblemetal ion reduction, NMs were also produced in solution inaddition to the NMs directly grown on the surface of MOs.Inevitably, the final products contain a mixture of both noblemetal–metal oxide nanohybrids and free NMs. This is unfavor-able for the properties of the resulting nanohybrids.66 In order tocircumvent this problem, Pearson et al. locally immobilizedphosphotungstic acid onto a TiO2 surface (commercial anataseTiO2 powder), which acted as a highly targeted photoactivereducing agent in the presence of UV light.67 By this method,

Fig. 2 (a) High angle annular dark field STEM of Au decorated ZnO nanotetrapods. (b) Top and cross-sectional (inset) SEM images of Ag/TiO2 nanotubecomposite arrays. TEM of (c) Au/TiO2 core/shell NPs, (d) Au/TiO2 yolk/shell NPs, and (e) Janus AuAg–Fe3O4 NPs. Reprinted with permission fromref. 55–59. Copyright 2016, Nature Publishing Group, 2010 Royal Society of Chemistry, 2015 American Chemical Society, 2011 Wiley-VCH and 2010Wiley-VCH.


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only noble metal ions directly adsorbed to the surface werereduced to nanoparticles thus eliminating the formation of freenoble metal nanoparticles. Cui et al. fabricated CeO2/Pd, Fe3O4/Au and Mn3O4/Ag nanocomposites with tunable NM particle sizeusing a robust one-step strategy.68 No additional reducer wasneeded as the redox reaction occurred between the metalhydroxide and noble metal ion. For example, CeO2/Pd compositeNPs were formed by the redox reaction between Ce(OH)3 andPd2+. This indicated that Ce(NO3)3 not only provided a source ofCe ions but also acted as a reducer. And the size of the NMscould be modulated by adjusting the injection rate of the noblemetal ion solution. This approach is simple, however it can bechallenging to separate out the particles easily. This problem isaddressed in a related study where single-metal NPs (Pd, Au, andAg) and bimetallic NPs (PtAg) decorated Fe3O4 composite NPshave been fabricated via a simple in situ reduction method usingFe3O4/SiO2 core/shell structures as support materials. The mono-disperse Fe3O4 NPs are synthesized through the classic thermaldecomposition of iron(III) oleate.69 Nanocomposites composedof magnetic cores enable a better separation method forrecycling the nanocatalysts from the reaction mixture using amagnet, especially compared with those of uncoated magneticmaterials.

Any residual reducing agent left after the process ofchemical reduction may degrade the electron properties ofcomposites. It is ideal then to develop a method in which areducer is not required. The photoreduction (PR) method70–72

is similar to chemical reduction, but a chemical reducing agentis not necessary in the preparation process since electronsphotogenerated from TiO2 or ZnO can act as a reducing agentunder irradiation with light of the appropriate wavelength(Fig. 3b).69 Specifically, electrons are excited into the conduc-tion band from the valence band of the MO under irradiation.These excited electrons can be used to reduce adsorbed noblemetal precursors into noble metal particles deposited on thesurface of MOs. The advantage of this method is that it requiresno additional reducer or stabilizing agents making it possibleto obtain pure composites without any impurities and achieve aclean interface between the MOs and adsorbed NMs.73 The sizeof the NMs can be controlled by varying the irradiation period,irradiation energy and precursor concentration.74,75 Using amulti-step PR method in which the addition of metal pre-cursors and photoreduction were repeated several times, thedesired metal loading onto MOs can be obtained.76 CommercialCeO2 powder decorated with Au NPs was successfully preparedby a PR method using a 400 W high-pressure mercury arc as theirradiation source.77–79 Analysis of the liquid phase after thephotodeposition revealed that the Au source was almost com-pletely deposited as Au metal on the CeO2 particles. Such resultsindicate that PR is a highly efficient method for the deposition ofNMs on various MOs.

Deposition–precipitation (DP)80–87 has been commonlyemployed to deposit well dispersed Au NPs on MOs. Normally,pre-grown MOs are dispersed into a Au precursor solution, suchas HAuCl4. The precipitates obtained after a period of stirringare then calcined to decompose the Au precursor to the metallicstate. The method to prepare the ZnO MO template involves afacile abrupt precipitation employing zinc nitrate and sodiumcarbonate as the precursors.88,89 The pH value of these solu-tions is a key factor determining the structures and propertiesof the resulting NM–MOs.90,91 Moreau et al. found that Au/TiO2

catalysts prepared by DP at pH = 9 produced the optimumcatalytic activity.92 Since the TiO2 support possesses negativesurface charges at this pH, it is unfavorable for the depositionof chlorine ligands which are detrimental to the catalyticproperties.93 It has also been demonstrated by the work ofTran et al. that the final pH of the preparation solution stronglyinfluences the Au particle size, Au deposition yield, chlorineremoval efficiency and consequently the overall catalystactivity.94 This was attributed to a more efficient chlorineremoval at higher pH leading to an improved dispersion ofAu NPs and thus higher catalytic activity. In addition to pH, thechoice of precipitation agent is also important. The mostcommon choice is NaOH in which the size of Au NPs can becontrolled by varying the solution pH and calcination tempera-ture. However, one problem with the use of NaOH is that not allthe Au in the growth solution deposits on the MO support.Some precipitates in free solution. By using urea as the pre-cipitation agent, all Au from the solution was deposited on the

Fig. 3 Strategy for the synthesis of TiO2 NPs decorated with NMs using (a)chemical reduction and (b) photoreduction. Reprinted with permissionfrom ref. 63 and 69. Copyright 2013 American Chemical Society and 2010Springer.


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MOs without precipitation in the free solution to achieve ahigher Au loading.95,96 In an interesting work by Naya et al., twosizes of Au NPs, 2 nm and 9 nm, were deposited on the surfaceof TiO2 NPs by a two-step route consisting of simultaneousdeposition–precipitation and chemical reduction. The resultingcatalyst composites exhibited high levels of visible-lightactivity.97 Some advantages of this approach include the abilityto produce small Au NPs (sizes of less than 10 nm) and awell-defined coupling between the Au NPs and the supportcan be obtained.89,98 Small Au NPs decorated on the surface ofMOs significantly increases the active sites for the adsorption ofreactants during the catalytic process. A well-defined couplingbetween the two components improves the separation ofphotoelectrons.

2.1.2 Properties. Depositing NMs on the surface of MOs isthe most common way to synthesize NM–MOs. Depositing NMson the surface of MOs is also one of the most efficient methodsto modify the surface of MOs, which directly influences theproperties of MO-based nanocomposites.99,100 As the size ofthe MO decreases to the nanoscale, a higher surface area isobtained which can lead to improved photocatalytic and photo-voltaic efficiencies. However, surface defects increase withincreasing surface area which is unfavorable for charge separa-tion and transfer.10,101 Given this problem, a series of strategiesfor the surface modification of MOs have been developed ofwhich the deposition of NMs is one attractive alternative toovercome these problems. For example, NM-decorated {221}faceted octahedral SnO2 nanocrystals have been synthesized asan effective strategy to enhance the sensitivity and selectivity ofmetal oxide-based gas sensors.102

In addition, the performance of MO-based nanohybrids inthe areas of energy and environmental remediation significantlyimprove after modification with NMs. Notable applicationsinclude photodegradation of organic pollutants,103,104 photo-catalytic hydrogen generation,105,106 photocatalytic reductionof carbon dioxide,107,108 photosynthesis of organic mole-cules,109,110 and solar cells111,112 among others.

Morphology is a key factor influencing the properties ofNM–MOs as different shapes contain a different number ofsites for the binding of NMs. This has an effect on the overallphotocatalytic and photovoltaic properties of NM-decoratedMO nanocomposites.113 To date, diverse morphologies of MOs,such as nanoparticles,114 nanospheres,115 nanoflowers,116

nanotubes,117 nanorods,118 nanowires119 and other morecomplex structures120,121 have been reported. These variousmorphologies oftentimes improve their photocatalytic andphotovoltaic properties, as well as provide suitable substratesto incorporate NMs. Thus various composites with differentmorphologies can be obtained by using different kinds of MOsas templates, such as Au-decorated TiO2 particles (Fig. 4a),122

Cu2O/Pd cuboctahedrons composites (Fig. 4b),123 TiO2/Ag nano-tube heterojunctions (Fig. 4c)124 and hollow toroidal ZnO/Aunanostructures (Fig. 4d).120 More importantly, the properties ofcomposite materials can be readily adjusted by controlling themorphology and structure type. Consequently, different appli-cations based on NM-decorated MO nanostructures can be

designed and synthesized. In one example, a three-dimensionalordered TiO2/Au nanostructure was fabricated through theassembly of thin shell-TiO2/Au hollow nanospheres. The struc-ture showed a visible-light-driven photocatalytic activity that isseveral times higher than conventional Au/TiO2 nanopowdersdue to multiple light scattering, large Brunauer–Emmett–Teller(BET) surface area and slow photon effect resulting from thisunique architecture.115 This nanostructure not only can beused for high efficiency photocatalysts, but also has applica-tions in fields requiring high surface area materials suchas water treatment, filtration and separations. Many similarperformance enhancing architectures like this are possible. It isclear that morphological engineering of MOs is an effectivestrategy to optimize the photocatalytic performance of compo-sites involving MOs.125

2.2 NM-decorated metal oxide nanoarrays

2.2.1 Preparation. Recently NMs have been used todecorate different kinds of metal oxide nanoarrays,126–141

including nanorods (Fig. 5a),142 nanotubes (Fig. 5b),143 nano-wires (Fig. 5c)144 and nanosheets (Fig. 5d).145 Several studieshave centered on loading NMs on metal oxide nanoarrays.Chemical reduction144,146 and photoreduction methods,147–150

which are used for depositing NMs on the surface of metaloxide NPs, can be also used to attach NMs onto metal oxidenanoarrays. By these techniques, the degree of NM coverage onthe metal oxide nanoarray surfaces can be readily tuned byadjusting the noble metal precursor amount and immersiontime. Au–Pd co-modified TiO2 nanotube films have been synthe-sized by simultaneous photo-reduction of Au and Pd precursorson TiO2 nanotube arrays.151 Compared to pure TiO2 nanotubearrays, the contaminant elimination rate (malathion pesticide)increased by 172% when using composite nanotube array photo-catalysts. The reasons for the improved photocatalytic activity

Fig. 4 (a) SEM image of TiO2/Au NPs. TEM images of (b) Cu2O/Pdcuboctahedron composites, (c) TiO2/Ag nanotube heterojunctions and(d) Hollow doughnut-like ZnO/Au nanocomposites. Reprinted withpermission from ref. 122–124 and 120. Copyright 2015 Wiley-VCH,2014 Wiley-VCH, 2015 American Chemical Society and 2012 AmericanChemical Society.


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might be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H2O2

due to the co-deposition of Au and Pd.One thing to be noted is that high coverage by NMs may lead

to the formation of composite nanoarrays bundles, especially inthe case of nanowires and nanoneedles.144,148 The main reasonfor this is the center of gravity of each composite nanowire ornanoneedle is not aligned with their neighbors due to the non-uniform decoration of NMs on the surface of metal oxidenanoarrays at high NM loading. Interestingly, well-alignedvertical ZnO nanowires can be connected with each other byAu nanowires to construct a novel cross-linked structure viathe photoreduction method.139,152 Due to the efficient photo-charge transfer from Au nanostructures to ZnO nanowires, thisnovel cross-linked ZnO/Au composite nanoarray showed improvedphotocurrent densities when compared to pure ZnO nanowirearrays and traditional Au NP-decorated ZnO nanowire arrays.

As for nanotube arrays, the critical challenge of NM deposi-tion is the difficulty of filling nano-sized cylindrical pores withlarge amounts of NMs without clogging the nanotubes.153

In situ growth of NMs in metal oxide nanotube arrays fromelectroless-deposition is simple and cost-effective. It can pro-vide a uniform coating of NMs on the surface of the outer andinner walls of TiO2 nanotube array.154 This is in contrast withchemical reduction and photoreduction methods, which typi-cally deposit NMs near the top of the nanotube and less on theinner parts of the arrays. In addition to this, highly dispersedAg NPs loaded into TiO2 nanotube arrays have been preparedvia an ultrasonic-aided photoreduction technique.143 The ultra-sonication provides additional driving force for the incorpora-tion of the AgNO3 solution into the pore networks and thesubsequent precipitation of Ag NPs.

Owing to their relatively flat two-dimensional geometry,some specialized preparation methods can be used to deposit

NMs on the surface of metal oxide nanoarrays such as sputter-ing deposition.155–159 Sputtering time is a key parameter todetermine the size of deposited NMs.157,158 The average size ofAg NPs can be increased by increasing the sputtering time. For alonger sputtering time, Ag NPs created interconnected clusterscovering the top-side of TiO2/Ag nanotube arrays.155 Since mostmetal oxide nanoarrays are formed on conductive substratessuch as silicon wafers, fluorine-tin-oxide (FTO) glass or metalsubstrates, pulsed electrodeposition (PED) can also be used toconstruct highly dispersed NMs on nanoarrays.131,160–164 Thedensity and size of NMs can be tuned by changing the electro-chemical parameters such as peak potential, deposition timeand pulse cycles. The average size of Ag NPs loaded on thesurface of TiO2 nanotube arrays can be precisely controlled from1.3 nm to 21.0 nm by carefully controlling PED parameters.131

This method can effectively suppress the agglomeration of NMsdue to the high nucleation rate under the pulse current at thenanotube entrances to prevent the pores from becomingclogged. Physical vapor deposition (PVD) can also be used todeposit NMs on the surface of metal oxide nanoarrays. Libudaet al. have grown Pt NPs at the tube openings of TiO2 nano-tubular arrays prepared by electrochemical methods using PVDin ultrahigh vacuum.165 Pd NPs were homogeneously distributedinside the tubes when the particle precipitation (PP) method wasused for deposition. The relationship between the nanostructureof TiO2/Pt and their adsorption kinetics was demonstrated usingin situ infrared reflection absorption spectroscopy and molecularbeam methods. Results indicated that the distribution of NPs inthe nanotubes led to significant differences in adsorption andsaturation behavior due to the differences in surface and gasphase transport resulting from the different nanostructures.Thus, it may be possible to tailor the transport processes incatalysts by controlling their structures.

2.2.2 Properties. Metal oxide nanoarrays are a specialmetal oxide group possessing vertically-aligned structures.1D-nanostructures share some common characteristics withtheir NP counterparts, such as quantum size effects. Howeverthey also possess advantages that are absent or difficult torealize in NP systems, such as direct electron transport paths.Compared to NPs, the vertically aligned 1D nanostructurearrays exhibit peculiar physical properties which persist afterthe deposition of NMs. First, the 1D geometry facilitates chargetransport and reduces the recombination of electron–hole pairsby providing a direct unidirectional electrical channel(Fig. 6a).166 In contrast, photo-generated electrons often recom-bine at the boundaries in particle systems. Second, the well-defined features of the arrays can trap light in the gaps leadingto multiple internal reflections (Fig. 6b).167 Thus, light scatter-ing and absorption can be greatly enhanced and reduce reflec-tion in the visible range to a very low level. Third, the separationof conventional powdered photocatalysts from both the growthsolution and treated water is difficult and energy consumptive.Thus, residual photocatalyst present in treated water may causesecondary pollution. Similarly, loose photocatalyst powderemployed in air purification is also inappropriate due to theadverse health effects associated with its inhalation.168,169

Fig. 5 SEM of (a) Au-decorated ZnO nanorod arrays, (b) Ag-decoratedTiO2 nanotube arrays, (c) Au-decorated ZnO nanowire arrays and (d)Ag-decorated CuO nanosheet arrays. Reprinted with permission fromref. 142–145. Copyright 2013 Elsevier, 2009 Elsevier, 2010 AmericanChemical Society, and 2014 Elsevier.


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In contrast, nanoarrays directly grown on substrates make itpossible for the easy recovery and reuse of photocatalysts.

2.3 NM/MO core/shell nanostructures

2.3.1 Preparation. In comparison with NM-decorated MOnanostructures, the preparation of NM/MO core/shell nano-structures is rather complex since there is typically a latticemismatch between the dissimilar noble metal core and metaloxide shell components.170–172 Normally, a ligand or surfactantis introduced between the NM core and MO shell to addressthis incompatibility. Polyvinylpyrrolidone (PVP) is one of themost commonly used surfactants and serves multiple rolesduring the formation of core/shell nanostructures. It was foundthat the surfactant PVP not only tuned the interfacial energies,but also prevented the growth and aggregation of ZnO.170

Sol–gel methods are an effective strategy to fabricateNM/MO core/shell nanostructures based on the introductionof ligand or surfactant molecules. Normally, a pre-made NMcolloid prepared by seeded growth reaction173 or facile redoxreaction174 is added to the metal oxide precursor solutioncontaining a specific ligand or surfactant, such as PVP, andthen core/shell nanomaterials are obtained by the hydrolysisand condensation of the precursor in the presence ofsurfactant-capped NMs. The morphology, structures and pro-perties of the resulting materials are dictated by the experimentalconditions and nature of the precursor and surfactant.175 Thereare several advantages associated with using sol–gel strategies.First, the introduction of a ligand or surfactant efficiently reducesthe interfacial energy between core/shell structures.170 Second, theshell thickness and core diameter are tunable, which is importantfor property and performance optimization. For example, Au/TiO2

core/shell NPs have been prepared using a controlled sol–gelprocess.176 The Au loading amount can be easy tuned by simplychanging the coating times. At the same time, a wide range ofTiO2 thicknesses can be obtained by changing the water contentand Au concentration. Highly dispersed Ag/TiO2 core/shell nano-structures can also be obtained using sol–gel methods withalmost all TiO2 NPs possessing a Ag core, which can be used asphotochromic and photocatalytic materials at the same time.174

Pd/Cu2O core/shell NPs have been fabricated using CuCl2 solutionand Pd nanocube precursor solutions in which the morphology ofCu2O and the size of composite NPs could be tuned by controlling

of the concentration of the CuCl2 and Pd nanocube solutions,respectively.177

Another common method to fabricate NM/MO core/shellnanostructures is the hydrothermal treatment of metal salts(e.g. TiF4) and colloidal NMs. Unlike in sol–gel methods,ligands or surfactants are not needed in hydrothermal methodsmaking it simpler. Yu et al. have fabricated flower-shapedAu/TiO2 and Pt/TiO2 core/shell NPs by a simple hydrothermalroute using TiF4 precursor in which the Au and Pt colloidal NPswere produced in a redox procedure using HAuCl4 and H2PtCl6

as noble metal precursors respectively.178,179 The concentrationof F� has a significant role in the morphology of the formedTiO2 shell. It was reported that the more perfect wedged-shapedmorphology and better crystallinity were present when theconcentration of the TiF4 solution was high. Microwaves areoften used to assist the formation of core/shell nanostructuresproduced via the hydrothermal route. Under microwave irradia-tion, high reaction temperatures are quickly reached allowingthe reaction time to be reduced.180,181

Multi-core noble metal/metal oxide shell nanocompositesare an important type of noble metal/metal oxide core/shellnanostructure which can be fabricated via several templatemethods (carbon, polystyrene and SiO2 spheres).182–184 Multi-Pd core/hollow CeO2 core/shell nanocomposites have beensynthesized using PVP-functionalized carbon nanosphere tem-plates as shown in Fig. 7.185 The hydrothermal treatment ofCe3+, and the removal of the carbon template and PVP stabilizercan all be achieved in the last calcination process in air. Thisresults in the formation of Pd/CeO2 core/shell nanostructures.Song et al. have fabricated Au/TiO2 core/shell nanostructureswith Au NPs embedded into the inner wall of the mesoporousTiO2 hollow spheres using sulfonated-polystyrene spheres (SPS)as a template.186

Using a block copolymer as a template is an efficient way tofabricate highly ordered core/shell NPs with controlled size,shape and composition. Recently, our group has developed aversatile strategy to fabricate Au/TiO2 core/shell NPs using star-like poly(4-vinylpyridine)-block-poly(t-butyl acrylate)-block-poly-styrene (P4VP-b-PtBA-b-PS) as a nanoreactor.57,187 First, a Auprecursor, HAuCl4 solution, was selectively incorporated in thespace occupied by the inner star-like hydrophilic P4VP blocks.The strong coordination between the metal moieties of theprecursor and the pyridyl group of the P4VP blocks leads to

Fig. 6 (a) Direct charge transport and (b) multiple internal reflections ofincident light in 1D nanoarrays. Reprinted with permission from ref. 166and 167. Copyright 2015 Royal Society of Chemistry.

Fig. 7 Schematic illustration of the fabrication of a Pd/CeO2 core/shellnanocomposite: (I) synthesis of carbon sphere coated with PVP template;(II) preparation of colloidal Pd nanoparticles from tetrachloropalladic acid;(III) fabrication of carbon–Pd–Ce(III) nanocomposites via a hydrothermaltreatment; (IV) preparation of Pd/hCeO2 core/shell nanocomposite withinner hollow space by a calcination process. Reprinted with permissionfrom ref. 185. Copyright 2013 American Chemical Society.


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the nucleation and growth of the Au core NPs. Subsequently, thePtBA chains of the PtBA-b-PS blocks that are situated on thesurface of the Au core are hydrolyzed into poly(acrylic acid) (PAA)chains. Similarly, the selective coordination of Ti(OCH(CH3)2)4

(TTIP; TiO2 precursors) in the compartment containing the PAAblocks facilitated the in situ nucleation and growth of the TiO2

shell via strong coordination interactions between the carboxylgroups of PAA and metal moieties of TTIP. This results in theformation of Au/TiO2 core/shell NPs as shown in Fig. 8.57 The corediameter and shell thickness of the resulting Au/TiO2 core/shellnanocomposite can be tailored by tuning the molecular weight ofP4VP and PtBA in the star-like template. It is worth nothing thatPtBA provided a template for the growth of TiO2 implying that thesynthesis of TiO2 is independent of the Au core. What this meansis that a lattice match between core and shell materials is not

necessary as the growth is non-epitaxial. This method can be usedto fabricate other core/shell nanostructures composed of twodissimilar materials. Furthermore, the PS chain outside the nano-reactors can be easily converted into carbon to form Au/TiO2/carbon core–shell–shell nanostructures after annealing in argongas for use in dye-sensitized solar cells (DSSCs) and other areas.

2.3.2 Properties. As discussed above, NMs deposited on thesurface of MOs is the most straightforward way to produce hetero-interfaces. Coating the surface of NMs with MOs to form NMs/MOscore/shell nanostructures is another promising way to integrate MOswith NMs. In contrast to the NM-decorated metal oxide nano-particle/nanoarray structures, the whole surface of the NM core isfully covered with a MO shell coating in core/shell nanostructures toform a 3D hetero-interface between the NM core and MO shell188–190

as shown in Fig. 9.170

Fig. 8 Schematic illustration of the synthesis route for (PS) Au/TiO2 core/shell NPs. Reprinted with permission from ref. 57. Copyright 2015 AmericanChemical Society.

Fig. 9 TEM images and photographs of metal/ZnO NPs that were synthesized from different noble metal cores: citrate-stabilized NPs, including (a) Aunanospheres; (b) Ag nanospheres; and (c) Pt nanospheres. PVP-stabilized NPs, including (d) Pd nanospheres; (e) Ag nanocubes (diameter = 150 nm); and(f) Ag nanowires. Insets show magnified views of typical NPs. Scale bar: 200 nm. Reprinted with permission from ref. 170. Copyright 2013 AmericanChemical Society.


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Pure NMs, such as Pd, Pt, Au and Ag NPs, have large surfaceenergies and tend to aggregate in solution which limits theirapplication in various fields.174,191,192 Depositing NMs on thesurface of MOs is an effective way to solve the dispersionproblem of NMs. However, in this strategy NMs are also directlyexposed to reactants, products, and the surrounding medium.Undesirable corrosion or dissolution of the NMs during theirpreparation and incorporation into device architectures leads toloss of the unique properties of the original NPs. By encapsulat-ing NMs with MO shells, it is possible to simultaneously preventNM aggregation and avoid undesirable corrosion.193–196

Thermal stability is an important property for many nano-materials. Usually, high temperature treatment damages themorphology and changes the size of NMs and MOs. Thisproblem is not easily avoided in NM-decorated metal oxidenanoparticle/nanoarray structures. For example, the anatasephase of TiO2 is unstable at high temperatures and starts totransform into the rutile phase. Considering that the anatasephase is desirable in photovoltaic devices, minimizing thistemperature-dependent phase transformation is essential.Encapsulating NMs with a metal oxide shell provides a physicalbarrier against sintering which can effectively slow down thegrowth of NMs and prevent the phase transformation of TiO2 athigh temperatures as well as protect other important MOphases at elevated temperatures. Kwon et al. have demon-strated that encapsulating Au NPs with a TiO2 shell can preventthe growth of Au crystals during heat treatment.197 Hence, thesize change of Au NPs in Au/TiO2 core/shell nanostructures ismuch smaller than that in pure Au NPs. Another reason for thestability is that Ti–O–Au bonds form at the interface of the NMsand MOs.171 It was found that no phase transition from anataseto rutile occurred when the Au/TiO2 core/shell NPs were sinteredat 900 1C.189 In contrast, the anatase phase for pure TiO2 startedto change to rutile when the calcination temperature exceeds500 1C. So heterostructures with NM cores and MO shells cansignificantly increase the thermal stability not only for NMs butalso for MOs. The excellent thermal stability of Au/TiO2 makes ita promising candidate for high temperature CO sensors for airquality monitoring systems in automobiles and some otherapplications involving high temperature environments.

The interfacial area of nanohybrids is critical to the propertiesof nanocomposites. Due to the interaction between the NM andMO components, NM–MOs may exhibit a combination of pro-perties from the two components and possibly further enhanceproperty tunability. NM-decorated metal oxide NP/nanoarraysprovide only a 2D interaction between the NMs and MOs whichleaves the majority of the NM surface uncovered.193 While, theNM/MO core/shell nanostructures provide intimate 3D contactbetween the NM core and MO shell which provides a largerinterfacial area. As a result, the direct interfacial electronictransfer process between the two components is strengthened.Au/Cu2O core/shell nanocomposites have been synthesized via afacile wet chemical approach in which the shell thickness andcore size can be precisely controlled.198 The incorporation of theAu core into the shell of Cu2O contributed to further enhance-ment of the plasmonic tunability due to the dielectric properties

of the Cu2O shell surrounding the Au core. NM/MO core/shellnanostructures can also be used to fabricate yolk/shell nano-structures. Rattle-like Au–Cu2O yolk–shell NPs with well-controlled spacing between the core and shell were formedthrough a symmetric hollowing (etching) process after theformation of core/shell nanostructures.

2.4 NM/MO yolk/shell nanostructures

2.4.1 Preparation. NM/MO yolk/shell nanostructures (rattle-type nanostructures) possess a hollow metal oxide shell with asolid NM core that can freely move within the shell. It is easy toencapsulate metal cores within SiO2 shells to form yolk/shellnanohybrids by selectively etching part of the noble metal core ofpreformed core/shell NPs. SiO2 hollow shell-based yolk/shellnanostructures have been widely reported, especially after thepioneering work by the Xia group.199–204 However, since theSiO2 hollow shell is insulating, it can only be used as astabilizer. The use of functional MOs, such as ZnO, Fe3O4

and NiO as the shell, adds other properties and functionalitiesto the overall architecture.205,206

Compared to SiO2, it is more challenging to produce yolk/shell nanostructures with other MO shells because of thecorrosion of most MOs during the etching process. As a result,SiO2 is widely used to synthesize NM/MO yolk/shell nanostruc-tures by serving as a sacrificial hard template. Typically, SiO2 isfirst coated on the surface of noble metal cores to form core/shell nanostructures. Next, a metal oxide shell is deposited onthe surface of the core/shell composites through the hydrolysisof precursors such as tetrabutyl titanate.207 Lastly, NaOHsolution is added to remove the SiO2 layer and form the noblemetal/metal oxide yolk/shell nanostructures.58,208,209 Normally,a final calcination is needed to crystallize the amorphous MOsas shown in Fig. 10a.208 The crystallinity of the hollow TiO2

shell, which is important for the H2 production rate in watersplitting applications, is easily controlled by changing thesynthesis and calcination conditions.210 In addition to this,the thickness and pore size of the TiO2 hollow shell also playessential roles in the catalytic reaction. Li et al. fabricated Aunanorod/TiO2 yolk/shell photocatalyst. The average pore sizeand thickness of hollow TiO2 are 5.1 nm and 24 nm respec-tively. At these sizes, the catalyzed water splitting proceedssmoothly and at high rates.207 Au/ZrO2 yolk/shell catalysts withhigh-temperature stability have also been synthesized usingSiO2 as a sacrificial template.211 Importantly, the inside Au coresize can be decreased after the Au NPs are covered with the SiO2

shell by treating Au/SiO2 with a NaCN leaching agent. The meanparticle size of the Au core reduced from 15 to 10 nm aftertreatment with NaCN. Au/SnO2 yolk/shell nanospheres withimproved sensing properties have been successfully synthesizedby also using Au/SiO2 nanospheres as sacrificial templates.212

The use of polyvinylpyrrolidone (PVP) as the stabilizing agentensured a uniform coating of SiO2 on the Au NPs. It is worthnoting that the key to this process is the use of Au particles richwith surface functional groups to which a significant amount oftemplate ions can adsorb.213


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Similar to SiO2, carbon can also be used as a hard templateto fabricate noble metal/TiO2 yolk/shell nanostructures.214 It ismore advantageous to use carbon templates compared to SiO2

since the removal of carbon and crystallization of MOs can beaccomplished simultaneously by thermal treatment. SomeMOs, such as ZnO and Cu2O, are sensitive to both acidic andalkali conditions. This makes it impossible to use SiO2 as atemplate to fabricate yolk/shell nanostructures. Thus carbon isan attractive alternative template for the synthesis of this kindof nanostructure. Li et al. successfully prepared Au/ZnO yolk/shell structures with an average diameter of about 280 nm andan average ZnO shell thickness of 40 nm by coating ZnO on thesurface of Au/carbon core/shell nanostructures followed byremoval of the sacrificial carbon shell though calcination athigh temperatures in air.215 In the area of chemical sensors forexample, Au/NiO yolk/shell composite NPs showing a highresponse to H2S have been fabricated via a precipitationmethod using carbon encapsulated Au core/shell NPs as asacrificial template.216 The maximum response of Au/NiOyolk/shell NPs was approximately 4 times higher than that ofbare NiO hollow nanospheres due to the hollow spaces thatallowed the accessibility of Au NPs to gas molecules.

In addition to hard templates, soft templates such as poly-(2-vinyl pyridine)-block-poly(ethylene oxide) (PVP-b-PEO) have

also been used to fabricate Au/TiO2 yolk/shell nanostructures asshown in Fig. 10b.217 This method is similar to using blockcopolymers to fabricate core/shell nanostructures. The differenceis the reduction of the Au core precursor is facilitated by a deepUV treatment, and the outer PEO blocks serve as the anchoringsites for the growth of the TiO2 shell thus leading to theformation of Au/TiO2 core/shell nanostructures. The internalcross-linked PVP-b-PEO copolymers are then removed by anotherdeep UV treatment to form Au/TiO2 yolk/shell nanostructures.This method is easy to control and is an alternative way toprepare other kinds of yolk/shell materials sensitive to pH.

2.4.2 Properties. Yolk/shell can be considered as a specialkind of core/shell nanostructure. The major difference is thevoid between core and shell in yolk/shell nanostructures, asshown in Fig. 11.198 Hence some properties of NM/MO yolk/shell nanostructures are the same as NM/MO core/shell nano-structures. For example, hollow metal oxide shells prevent NMsintering into large particles during high temperature calcina-tion as well as help maintain their structural integrity. In thework of Yin et al., the size and shape of Au cores in Au/TiO2

yolk/shell nanostructures were almost the same upon heattreatment up to 775 K. When the mixture of Au NPs and P25was exposed to the same temperature; Au NPs grew from 10 nmto 50 nm with apparent sintering. The reason for the stability is

Fig. 10 Proposed formation scheme for Au/TiO2 yolk/shell nanostructures based on (a) hard template approach and (b) soft template approach.Reprinted with permission from ref. 208 and 217. Copyright 2015 American Chemical Society and 2011 Royal Society of Chemistry.


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likely due to the formation of a sodium titanate phase duringthe etching step using NaOH.58

In addition to the properties of core/shell nanostructures,yolk/shell nanostructures also have some unique characteris-tics. First, since the inside core is isolated and has less contactwith the outside hollow shell, any contact interference from theshell is minimized. As a consequence, yolk/shell structures areexcellent for modeling the catalytic properties of NMs when theshell is an inert material such as SiO2.218 Furthermore, becauseof the detachment from metal oxide shell, the internal noblemetal core provides more exposed active sites for catalyticreactions of diffused species.207,215,219 Second, the hollow shellsare normally porous, which reduces diffusion resistance andincreases the accessibility of reactants to the catalyst core anddiffusion of the catalyzed products out of the space.220

The methods to synthesize NM/MO yolk/shell nanostruc-tures are still limited, especially for hollow metal oxide shells.Both hard templating and soft templating approaches arerelatively complicated and involve long reaction times whichgreatly limit the application of yolk/shell nanostructures.Therefore, a facile, effective and general method is needed forthe preparation of yolk/shell nanostructures.

2.5 Janus noble metal–metal oxide nanostructures

2.5.1 Preparation. Janus NPs refer to a special kind ofmaterial whose compositions contain two or more distinct

physical and chemical properties. Since the concept describedby De Gennes in 1991,221 various efforts been focused onfabricating different kinds of Janus NPs.222–227 However, thereare only a few publications about Janus noble metal/MOsnanostructures.228–232 The preparation methods can be mainlydivided into three groups. In the first, various morphologies ofNMs, such as nanorods and nanospheres, are fabricated. Next,a coating of MO on one side of the NM is applied. In Seh’s work,short and long Au nanorods are first synthesized using seed-mediated growth. Then, TiO2 is grown anisotropically on oneside of the nanorods by controlled hydrolysis of titaniumdiisopropoxide bis (acetylacetonate) because of its slow hydro-lysis rate.233 It is worth mentioning that the manner of additionof the Au precursor was important to the final structure of theproducts. It was found that only when all of the Au precursorsolution was added all at once that the Janus geometry wasobtained. Pradhan et al. first fabricated Au NPs. Then the twohemispheres of the particles were decorated with hydrophobicand hydrophilic ligands, respectively. Finally, TiO2 NPs wereformed on one side of the Au NPs by a surface sol–gel process,leading to the formation of Janus Au–TiO2.234 Yin et al. havefabricated asymmetric hexagonal pyramid-like Au–TiO2 nanostruc-tures via a similar method using Au nanocrystals and zinc acetatedihydrate as the starting materials.235 Sun et al. have fabricatednon-centrosymmetric Janus Au–Fe3O4 (Fig. 12a), Ag–Fe3O4

(Fig. 12b) and AuAg–Fe3O4 (Fig. 12c) using a general approach.

Fig. 11 Formation of rattle-like Au/Cu2O yolk–shell NPs. Reprinted with permission from ref. 198. Copyright 2015 American Chemical Society.


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MOs were grown over the pre-synthesized noble metal seeds bythermal decomposition of a metal carbonyl followed by oxida-tion in air.59 This robust method could be generalized to otherheterogeneous nanomaterials for various functional applica-tions. As the Fe(CO)5 used in Sun’s method is high toxic,Dong et al. fabricate Janus Au–Fe3O4 via a similar strategyby replacing highly toxic Fe(CO)5 with a safe Fe precursor (ironoleate).236

In the second approach, NMs are attached on one side ofpre-made MOs to form Janus hybrid structures. For example,ZnO was first fabricated via solvothermal synthesis. Then AuNPs were formed on the vertices of ZnO via treatment ofHAuCl4 with ultraviolet (UV) illumination to obtain Au–ZnOpyramid nanocomposites.232 The robust photo-reductionmethod reported here is generally applicable for site-specificgrowth of NMs on various polar semiconductor crystals.

In the last group, the mixture of both MO and NM precur-sors is directly treated to form the respective MOs and NMstogether. Janus structures are formed from the coalescence ofthe two components. Janus Au–ZnO has been prepared bythermal treatment of a mixture of Au and Zn precursors.230

Specifically, the mixture of HAuCl4 and zinc stearate oleylaminesolution was heated to 120 1C for 15 min followed by treatmentat 280 1C for 1 hour with the aid of 1-hexadecanol surfactant.Au–ZnO hybrid nano-multipods were obtained with a Au core ofabout 7–9 nm in diameter and ZnO rods with lengths from13–30 nm. Well-defined interfaces between ZnO and Au havebeen formed via this method, while in some other reports NMsare simply attached onto the ZnO surfaces. It was also foundthat the morphology of Au–ZnO hybrids could be controlledselectively by using different surfactants. Au–ZnO petal-like andurchin-like core/shell nanoflower, nanomultipod and nano-pyramid structures were also achieved using 1,2-dodecandiol,triphenyl phosphine, 1-hexadecanol and a mixture of 1-hexa-decanol and dibenzylether surfactants, respectively.

2.5.2 Properties. Many heterogeneous nanostructures canbe classified as Janus nanostructures, such as normal Janusnanostructures (Fig. 13a),208 branched nanostructures (Fig. 13b),231

linear nanostructures (Fig. 13c),231 and nanopyramids (Fig. 13d).235

Generally, it is more complicated to fabricate Janus nano-structures compared to their single-component counterpartsdue to the large lattice mismatch between dissimilar compo-nents. However, much attention has been given to Janus

nanostructures because of the combination of different nano-materials at a small junction point. This leaves the otherfunctional regions exposed to capitalize on their electric, mag-netic, optical, optoelectronic, biological and catalytic applica-tions.224,225 Janus noble metal–metal oxide nanostructurespossess much higher catalytic activity because of the directexposure of the NM core to reactants on one side. Meanwhile,due to the protection from the MO coating on the other side,Janus nanostructures also show long-term stability in compar-ison with bare Au NPs. Seh et al. fabricated Janus, eccentric andconcentric geometries of Au/TiO2.233 Both the Janus and con-centric geometries are energetically stable structures, and JanusAu–TiO2 NPs demonstrated the highest catalytic reduction of4-nitrophenol to 4-aminophenol among the three nanostruc-ture geometries.

3. Applications of NMMOs

As previously discussed, different NM–MO nanostructurespossess a number of unique properties. Consequently, variousapplications in catalysis, solar cells, biotechnology and optics

Fig. 12 HRTEM images of (a) Au–Fe3O4, (b) Ag–Fe3O4, and (c) AuAg–Fe3O4 NPs. Reprinted with permission from ref. 59. Copyright 2010 Wiley-VCH.

Fig. 13 TEM of (a) Janus Au–TiO2, (b) branched Au–ZnO, (c) linearAu–ZnO and (d) Au–ZnO nanopyramids. Reprinted with permission fromref. 208, 231 and 235. Copyright 2011 Wiley-VCH, 2013 Royal Society ofChemistry and 2011 American Chemical Society.


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have been developed. In this section, two basic mechanisms forthe enhanced photochemical reactivity of NM–MOs are brieflydiscussed. Then, solar energy related applications, includingphoto-degradation of organic pollutants, photocatalytic hydro-gen generation, photocatalytic CO2 reduction, dye sensitizedsolar cells and perovskite solar cells based on different NM–MOnanostructures are introduced.

3.1 Mechanisms of high performance applications

It is useful to understand the physical mechanisms for theenhancement of various properties when NMs and MOs arecombined. The two main phenomena involved in the improvedphotoreaction are localized surface plasmonic resonance(LSPR) and Schottky barrier formation. Both of these arediscussed in the following section.

3.1.1 Localized surface plasmon resonance (LSPR). LSPRcan be described as the collective oscillation of free electronsinduced by incident light inside NMs.237 When the frequency ofincident light matches the natural frequency of free electronsoscillating against the restoring force of positive nuclei, theoscillation amplitude reaches a maximum. The resonantphoton wavelength is called the LSPR wavelength, which isdifferent for different NMs, and is also tuned by varying the sizeand shape of NMs, as shown in Fig. 14.29

As to NM–MOs, a direct benefit is that the light-absorptionspectral ranges of TiO2 and ZnO can be easily extended tovisible and even near-infrared regions by the LSPR effect ofNMs. Liu et al. reported wide absorption spectra of TiO2 byintegrating Au nanorods as antennas.238 The plasmon reso-nance absorptions of nanostructures can be tuned from 630 nmto 810 nm by manipulating the aspect ratio of the Au nanorods.

3.1.2 Schottky barrier. The formation of the Schottkybarrier at the NM–MO interface is highly dependent on thework function of the NM (FM) and the electron affinity of theMO (XSM). Normally, the value of FM is larger than that of XSM,where XSM is the energy difference between the minimumconduction band (CB) and the vacuum (Vac) energy. Also theFermi level (EF) of NMs is typically lower than that of MOs as

shown in Fig. 15.239 When the NMs and MOs come into contact,the electrons in the MO are transferred to the NM until theyreach equilibrium. This results in the bending of the CB(i.e. Schottky barrier FB). More importantly, this charge redis-tribution leads to a built-in electric field at the interface, whichinhibits the recombination of photogenerated charges andimproves the performance of devices for solar energy-relatedapplications.239–245 The formation of the Schottky barrier inZnO/Au nanocomposites has been investigated using time-correlated single-photon-count (TCSPC) spectroscopy.246 Com-pared to pure ZnO NPs, a slower fluorescence decay associatedwith the electron recombination process was noticed in theZnO/Au system. This confirmed the blocking of electron–holerecombination by the Schottky barrier formation.

3.2 Photo-degradation of organic pollutants

Photo-degradation of organic pollutants using MOs is a costeffective way to remove dyes from industrial water. The processof photo-degradation of organic pollutants can be described asfollows: under illumination of incident light with photonenergy higher than the band gap of MOs, the electrons in the

Fig. 14 (a) Normalized extinction spectra of different NMs (Ag, Au and Cu). (b) Normalized extinction spectra of different morphologies of Ag (wires,spheres and cubes), and (c) Normalized extinction spectra of different sizes of Ag. Reprinted with permission from ref. 29. Copyright 2011, NaturePublishing Group.

Fig. 15 Formation of the Schottky barrier between MOs and NMs (a)before contact and (b) after contact. Reprinted with permission fromref. 239. Copyright 2015 Royal Society of Chemistry.


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valence band (VB) are injected into the conduction band (CB)leaving behind the same amount of holes in the VB. Theelectrons trapped by O2 dissolved in solution form �O2

� super-oxide radicals which subsequently transform into �OH radicals.Meanwhile, photo-generated holes react with H2O adsorbed onthe surface of MOs to produce �OH. Both �O2

� and �OH arehighly reactive and can completely degrade most organic wastematerials into low toxicity inorganic small molecules. However,the use of bare TiO2 or ZnO is limited due to their wide bandgap and the high recombination rate of photogeneratedcharges. Coupling with NMs is an emerging strategy to improvethe degradation rate of organic pollutants under visible lightconditions.

3.2.1 NM-decorated metal oxide NPs. NM-decorated MOnanostructures are the most widely used photo-catalysis com-pared to other nanostructures of noble metal–metal oxidecomposites. The photocatalytic efficiency depends on severalparameters such as the NM loading amount and the morphologyof MOs.

The amount of NM loading on the surface of MOs directlyaffects the photo-catalytic ability of the composites. In general,there is an optimum amount of NM loading as shown inFig. 16a in which the optimum loading peaks at 5% anddecreases at higher loading.124 The photocatalytic activity initi-ally improves with increasing loading up to the optimumcontent. It is widely accepted that even at a very low loadingamount, the NMs can greatly enhance the photocatalytic per-formance of composites.240 The enhancement is partly due tothe increased optical absorption and improved charge separa-tion. When the NM density is low, the separation of electronsand holes improves with an increase in NM loading up to someoptimal amount.247 However, a further increase in loadingbeyond this critical point is detrimental to the photo-catalyticactivity. The reduction in performance can be attributed to thelight-shielding effect caused by the excessive NMs, the aggrega-tion of the NMs at high loading, the contact obstructionbetween the MO core and organic pollutants, and the increas-ing number of recombination centers for photo-generated

charge carriers.248,249 Fortunately, the loading amount of NMscan be easily modulated by controlling the concentration ofnoble metal precursor,124 reaction time, temperature, anddeposition method.250

The morphology of MOs also plays a significant role in thephotocatalytic activities of nanohybrids because of differentpore sizes, surface area and electron transport paths for differentshapes. For example, flower-like nanocomposites possess highercatalytic efficiency due to the large pore sizes, large BET surfacearea and multiple internal light reflections.251 Compared to NPs,nanorods, nanowires and nanotube-based hetero-structuresshow enhanced performance due to the one-dimensional struc-ture favoring charge transportation. Wang et al. prepared Aucoated ZnO nanorods and exhibited complete photo-catalyticdegradation of rhodamine B within 15 min as shown inFig. 16b.252

3.2.2 NM-decorated metal oxide nanoarrays. Metal oxidenanoarrays have been recognized as attractive photo-catalyticmaterials because of their high surface area-to-volume ratio,efficient charge transport, excellent photo-sensitivity and opticalanti-reflection ability.253 The photocatalytic efficiency of metaloxide arrays can be significantly increased by decoration withNMs. This is attributed to the efficient charge separation andlight absorption. ZnO nanorod/Pt and ZnO nanorod/Ag hetero-nanostructure arrays have been fabricated using ZnO nanorodarrays on a zinc substrate as a template. These arrays showedimproved photo-catalytic activity in the decomposition of RhBdye and excellent photo-catalytic recycle lifetime.254 Ag and AuNPs with a diameter of 10 � 2 and 28 � 3 nm were depositedon the surface of TiO2 nanotube arrays via photo-depositionand sputtering deposition, respectively.255 The photo-catalyticactivities of different nanocomposites (TiO2 nanotube arrays, flatTiO2, Ag-decorated TiO2 nanotube arrays, Au-decorated TiO2

nanotube arrays, Ag-decorated flat TiO2 and Au-decorated flatTiO2) were evaluated by the decomposition of Acid Orange 7(AO7). It can be seen that TiO2 nanotube arrays exhibit a higherdecomposition rate than that of flat TiO2. Moreover, comparedto the unloaded TiO2 nanotube arrays, both Ag-decorated TiO2

Fig. 16 (a) Effect of Ag content on the photocatalytic activity of Ag/TiO2 nanotubes, and (b) time-dependent optical absorbance spectra for the RhBsolution in the presence of ZnO/Au for different durations. Reprinted with permission from ref. 124 and 252. Copyright 2015 American Chemical Societyand 2009 American Chemical Society.


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nanotube arrays and Au-decorated TiO2 nanotube arrays exhib-ited improved photo-catalytic efficiency due to the formation ofSchottky junctions between the NM and TiO2 nanotubes. Au, Agand Pt NPs were deposited on TiO2 nanotube arrays via a facilelayer-by-layer self-assembly adsorption technique which exhibitedversatile photo-catalytic and catalytic reduction capabilities.256

The photo-catalytic performance of composite nanoarrays variedfor different deposited NMs and the kinetic rate constantsfollowed the order: Pt/TiO2 4 Au/TiO2 4 Ag/TiO2, which isconsistent with the results of the photo-currents of compositenanoarrays deposited with different NMs.

The NM loading amount in the composite nanoarrays is acrucial factor in the enhanced photocatalytic properties. Thephotocatalytic activity of TiO2/Au nanotube composite arrayswith different amounts of Au NPs and 3-mercaptopropionicacid (MPA) ligand have been tested.129 From the results ofphoto-luminescence (PL) and electron spin resonance (ESR),the amount of active species (�O2

� and �OH) increased after thedeposition of Au NPs. This leads to improved photo-catalyticproperties in Au-decorated TiO2 nanotube arrays. In addition,the pollutant degradation rate of composite nanoarrays wasaffected by the amount of Au NPs decorating the surface as shownin Fig. 17a.129 The highest degradation rate (0.0194 min�1) wasachieved at Au loading of 1.14 wt%. It is important to find theoptimum loading amount of NMs by tuning the preparationparameters.

Powder photo-catalysts, possessing high surface energy,easily aggregate and are hard to separate and recycle duringthe photocatalytic process.169,257,258 Since metal oxide nano-arrays are grown on substrates such as copper foil and fluorine-tin-oxide (FTO) glass, it is easy to recover and reuse the metaloxide nanoarray-based composite photo-catalyst without theneed for laborious centrifugation or filtration. Using thioctic acidas a molecular linker, high density, uniform and aggregate-free AuNPs were deposited on the surface of ZnO nanorod arraysprepared on a conductive substrate.135 The concentration of Auprecursor solution is a key factor in the deposition of Au NPs. Itaffects the final photocatalytic activity of composite nanoarrays.Under UV irradiation, the optimal Au-decorated ZnO nanorodarrays exhibited a photocatalytic rate 8.1 times that of bare ZnOnanorod arrays due to a reduction of electron–hole recombinationand the presence of directed charge transportation. The stabilityof photo-catalysts was tested by the photocatalytic degradation ofRhB for 15 cycles. A 90% degradation rate could be achieved after15 photocatalytic cycles, as shown in Fig. 17b.135

Due to their direct growth on a substrate, NM–MO nano-arrays can also be used as a surface-enhanced Raman Spectro-scopy (SERS) substrate. Consequently, there has been work oncombining photo-catalysis and SERS in what are called self-cleaning SERS substrates and recyclable SERS substrates.259,260

Multifunctional Au-decorated TiO2 nanotube arrays have been fab-ricated using in situ photo-deposition and hydrothermal methods.261

Fig. 17 (a) Comparison of photo-catalytic decomposition for Au NP–TiO2 nanotubes with different loading percentages of Au NPs. (b) Cyclicdegradation of RhB using Au-decorated ZnO nanorod arrays. (c) Performance of M/TiO2 (M = Au, Pd, Pt) core/shell nanocomposites and TiO2 P25 inthe photocatalytic degradation of RhB under the irradiation of visible light. (d) UV-vis spectra recorded during the photocatalytic degradation of RhBusing different catalysts. Reprinted with permission from ref. 129, 135, 193 and 230. Copyright 2008 Elsevier, 2013 Wiley-VCH, 2011 American ChemicalSociety and 2014 Royal Society of Chemistry.


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The SERS enhancement intensity induced from the compositenanoarrays at 1649 cm�1 was 5 and 60 times higher than that ofpure Au and TiO2 nanotube arrays respectively. It is worth nothingthat the composite arrays can clean themselves by photo-catalyticdegradation of target molecules adsorbed to the substrate andreactivated under UV irradiation in about 20 minutes. After thecomplete degradation of adsorbed organic pollutants, the compositenanoarrays can be recycled as SERS substrates. In addition tothe TiO2/Au nanotube arrays, Ag-decorated TiO2 nanograss,260

Au-decorated ZnO nanosheets262 and Au-decorated ZnOnanorods144 were also found to act as self-cleaning SERSsubstrates. These unique nanostructures offer new opportunitiesto reduce the cost of the degradation of organic pollutants.

3.2.3 NM/MO core (yolk)/shell nanostructures. Althoughthe deposition of NMs on the surface of MOs is an efficientway to enhance their photo-catalytic activity, the corrosion anddissolution of NMs during the photocatalytic process is hard toavoid.263 NMs fully encapsulated by MOs to form NM/MO core/shell nanostructures are an efficient way to overcome thisdrawback.264,265 Ag nanowires have been coated by TiO2 toform Ag/TiO2 core/shell nanostructures through a hydrolysisreaction and subsequent thermal treatment.265 The core/shellnanostructures showed strong visible-light absorption becauseof the plasmon resonance of Ag nanowires, and thus demon-strated much higher visible-light photo-degradation rates thanbare TiO2 NPs and Ag nanowires. More importantly, due to thepresence of an anatase TiO2 shell, the Ag nanowires wereprotected from oxidation during the catalytic process whichenables long-term stability.

Shell thickness plays a key role in the photo-catalytic activityof core/shell nanostructures. Research into the effects of thethickness of ZnO shells on the photo-catalytic activity ofAu/ZnO core/shell NPs has been reported.266 The photo-degradation efficiencies of Au/ZnO with different ZnO shellthickness were evaluated by the degradation of methyleneorange solution. It was found that the efficiency increased withan increase in shell thickness due to the presence of fewersurface states and reduced probabilities for recombination ofphoto-generated charges. This was confirmed by the heavilyquenched PL emission peak of Au/ZnO composites with thickershells. Similar results were demonstrated in the work ofButburee in which NM/TiO2 core/shell nanostructures werefabricated with well-controlled TiO2 shell sizes using a two-step surface modification process.267 The core/shell nanostruc-tures with a TiO2 thickness of 75 nm produced much higherphoto-catalytic activities than those with 25 and 50 nm shellsbecause of the improved optical absorption and larger surfaceareas. The type of NM and light source also play an importantrole in the photo-catalytic enhancement of core/shell nano-structures.268 Different NMs (Au, Pd, Pt) have been incorporatedinto MOs to form core/shell composite photo-catalysts.193 Theincorporation of NMs significantly increased the visible-lightphoto-catalytic activity of TiO2 due to the plasmonic effect andimproved separation of photo-generated charge carriers. How-ever, under UV light irradiation, pure TiO2 showed higheractivity than NM/TiO2 core/shell nanostructures. The reason

was that the introduction of NMs increased the visible-lightabsorption, but decreased the UV absorption intensity of core/shell nanostructures. Moreover, due to the different electrontrapping capabilities of Au, Pd, and Pt, different photo-catalyticefficiencies were observed: Pd/TiO2 4 Pt/TiO2 4 Au/TiO2 4 P25as shown in Fig. 17c.

As a photo-catalyst, NM/MO yolk/shell nanostructurespossess two advantages compared to NM/MOs core/shellnanostructures. One is the multiple reflections of incident lightin void spaces, which increases the efficiency. The second is thefast diffusion of both reactants and products through theporous hollow shell increasing the photo-catalytic rate.269–271

Though yolk/shell nanostructures have attracted great attentiondue to their fascinating properties, to the best of our knowledgeno work has focused on the photo-catalytic degradation appli-cations of noble metal/metal oxide yolk/shell nanostructures.

3.2.4 Janus noble metal–metal oxide nanostructures.Although novel Janus noble metal–metal oxide nanostructureshave shown some interesting properties, the study of theirphoto-catalytic performance is still limited. Nanopyramids arethe most researched photo-catalyst Janus noble metal–metaloxide nanostructures due to the large number of exposedcrystal facets and superior crystallinity. Au/ZnO hybrid nano-pyramids were fabricated as photo-catalysts,.235 RhB can becompletely removed by Janus Au–ZnO in as little as 10 minutes.It was found that the hybrid NPs demonstrate improved photo-catalytic activity over pure ZnO under UV-light irradiation dueto the homogeneous decoration of Au NPs and efficient chargetransfer between Au and ZnO.272,273 A similar result was alsoobserved in Yao’s work.232 Au NPs were selectively formed onthe vertices of ZnO nanopyramids when a mixture of HAuCl4

solution and ZnO nanopyramids was treated under UVirradiation. Due to the efficient electron–hole separationachieved in the Au–ZnO nanopyramids, the photo-catalyticefficiency of composite nanopyramids was much higher thanthat of bare ZnO nanopyramids. In addition, compared toother Au–ZnO nanostructures, such as petal-like nanoflowers,urchin-like nanoflowers and nanomultipods, Au–ZnO nano-pyramids also exhibited the highest photo-catalytic efficiencyin the degradation of RhB under UV irradiation, as shown inFig. 17d.230 The reason for the enhanced photocatalyticefficiency is that the nanopyramids possess exposed polar{001} facets which have the highest photo-catalytic activities.In addition, high degrees of crystallinity with fewer defects alsocontribute to its enhanced photocatalytic performance.

3.3 Photocatalytic hydrogen generation

Photovoltaic cells are the most direct way to capture solarenergy, but the electricity converted from solar energy mustbe used immediately or stored in some form.12 Hydrogen is onekind of alternative energy resource that is both renewable andenvironmentally friendly. Since the pioneering work of Hondaand Fujishima on photo-catalytic H2 generation on TiO2 elec-trodes in 1972,274 it has been recognized as a promisingstrategy for storing solar energy in chemical bonds withoutthe production of harmful byproducts.275 The mechanism for


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photo-catalytic H2 generation is similar to that for photo-catalytic degradation including the absorption of solar light,generation of electrons–holes and catalytic reactions for H2 orO2 evolution. The difference between them is the requirementof a semiconductor band gap. For H2 generation, the bottom ofthe CB should be lower than the H+/H2 redox couple (0.0 V vs.NHE) and the top of the VB should be higher than the H2O/O2

redox couple (+1.23 V vs. NHE).276 It is hard for a singlesemiconductor to meet all the requirements for photocatalyticH2 generation. Hence, co-catalyst composites have been exten-sively studied. NM–MO nanocomposites used as co-catalystshave received a great deal of attention because the incorpora-tion of NMs can broaden the absorption range to includevisible light and reduce the recombination of photo-generatedelectrons and holes.

3.3.1 NM-decorated metal oxide NPs. Au NPs deposited onthe surface of TiO2 have been recognized as an effective way toenhance the H2 generation rate of TiO2. The H2 generationefficiency depends on several parameters including Au NP size,Au NP loading, the polymorphic composition of the TiO2

support (pure anatase, brookite, rutile or biphasic combina-tions), type of sacrificial agent, the wavelength of incident light(UV or visible light), the calcination temperature and the pH ofthe suspension.105,277–288 Among these parameters, controllingthe size and loading mount of Au NPs has attracted muchattention.

One of the key functions of the Au NP size during the H2

generation process is in determining the reduction potentialsof the electrons transferred to the TiO2 conduction band.284

Using the C60/C60� redox couple as a probe for the determina-tion of the Fermi level, it was found that the Fermi level of TiO2/Au composites shifted 20, 40 and 80 mV to negative potentialfor 8 nm, 5 nm and 3 nm Au NPs, respectively.289 A greaternegative shift in the Fermi level is induced in smaller Au NPswhich can facilitate better charge separation. TiO2/Au hetero-structures with two different Au NP sizes of (4.4 � 1.7 nm and67� 17 nm) have been fabricated as catalysts using deposition–precipitation and photo-deposition methods. The amount of H2

generation for TiO2/Au with small particles was about 20 timeshigher than that for TiO2/Au with large particles under l 4400 nm excitation due to the greater negative shift in the Fermilevel as shown in Fig. 18a.284 Li et al. investigated H2 generationvia photocatalytic reforming of methanol on TiO2/Aucomposites.277 The rate of H2 production was greatly increasedand the concentration of byproduct CO decreased with thereduction of the Au particle size, because smaller Au particlesswitch the decomposition reaction mainly to H2 and CO2

products while suppressing the formation of CO and H2O.The Au loading amount is another key parameter that affects

the efficiency of TiO2/Au composite catalysts. Since varying theloading amount affects the size, morphology, coverage anddispersion of Au NPs, an optimized Au loading amount isrequired to achieve maximum H2 production.105 Typically, thereaction rate increases with an increase in Au NP loading up toa point. A further increase in Au loading leads to a decreasein the H2 evolution rate. For different kinds of TiO2/Au

composites, the optimized amount for maximum H2 generationefficiency is not the same. Silva et al. fabricated Au-decorated P25TiO2 composites with Au loading in the range 0–2.2 wt%. Thehighest initial H2 generation rate (4 � 10�2 mL min�1) andlargest final H2 volume (4 mL) at 3 h were obtained for thecatalyst with 0.25 wt% Au loading. Considering the low Auloading in such composite materials, it can be considered alow cost method for improving H2 generation rates.105 In thework by Jovic, Au supported P25 TiO2 composites with Auloading 0–10 wt% were used as photo-catalysts for H2 genera-tion. A sharp PL intensity decrease in composite photo-catalystwas observed even at low Au loading, suggesting that a smallamount of Au loading can significantly suppress the recombina-tion of photo-generated charges. The highest rates of H2 produc-tion were achieved at Au loadings of 0.5, 1 and 2.0 wt%, whichsuggests that photo-catalytic performance was independent ofAu loading over the 0.5–2 wt% Au range for this formulation(Fig. 18b).282 As a consequence, the photocatalytic activity can besimply manipulated by varying the Au NP size or loading amountthus enabling a controllable solar-to-fuel energy conversion.

The effect of the TiO2 polymorph on H2 generation byAu-decorated TiO2 is also important. Due to the fast recombi-nation of photo-generated electrons and holes, rutile TiO2

generally has poor photo-catalytic activity. While anatase TiO2

often shows much higher photo-catalytic activity due to longercharge carrier lifetimes.290 P25 is a commercially availabletitania mixture with around 80% anatase and 20% rutile byweight. The activities of Au-decorated TiO2 composites pre-pared with different kinds of TiO2 follow the order P25TiO2/Au 4 anatase TiO2/Au 4 rutile TiO2/Au.285 The highestreaction rate for Au-decorated P25 TiO2 was attributed to theimproved electron transport across the anatase–rutile interface.The low H2 generation rate of rutile TiO2/Au is because of therapid electron–hole recombination rates. In Murdoch’s work,the H2 production rate for Au-decorated anatase TiO2 wasalmost 100 times higher than that of Au-decorated rutile TiO2

as shown in Fig. 18c.279 The average size of the Au particlesdecorated on anatase TiO2 are consistently smaller thanthose supported on rutile TiO2. This may contribute to thehigher hydrogen yield for anatase TiO2-based composite photo-catalysts.

Under UV and visible light, both of the H2 generation rateand reaction mechanism of Au-decorated TiO2 nanocompositesare different.281 It is known that TiO2 can only absorb light witha wavelength less than 380 nm. So under UV light, photoelec-trons are generated from TiO2 and then transferred to Au NPsfor H2 evolution. While under visible light irradiation, Auparticles are excited via plasmon resonance to produce elec-trons which are then injected into the CB of TiO2 leading to H2

generation. Qian et al. have suggested a distinct mechanism forirradiation under visible light.284 It was found that no H2 wasgenerated by small Au NP-decorated P25 because of the lowplasmon absorption intensity under l 4 435 nm irradiation.While under l 4 400 nm light excitation, small Au–P25nanocomposites showed apparent H2 generation. It is sus-pected that under excitation of light with a wavelength larger


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than 435 nm, electrons generated from Au NPs played a majorrole in H2 generation. While, under l 4 400 nm light irradia-tion, the photoexcited electrons come from TiO2 itself. The H2

yield for Au-decorated TiO2 NPs was evaluated under differentlight sources. Under irradiation by both UV and visible light,the H2 generation rate was much higher than that under UVlight alone because both the Au NPs and TiO2 could beactivated and contribute electrons to the enhancement ofphoto-catalytic efficiency. There was no H2 generation oncomposite photo-catalysts under irradiation of visible light onlyas shown in Fig. 18d.286

In addition to the parameters discussed above, some otherfactors also play an important role in determining photo-catalytic activity. The type of sacrificial agent, the presence ofa co-catalyst, the calcination temperature, the exposed surfacefacets of MOs as well as the pH of the suspension also cangreatly impact the final photo-catalytic performance of theresulting nanocomposite.105,285,287,291–293

3.3.2 NM-decorated metal oxide nanoarrays. Metal oxidenanoarrays composed of nanorods, nanowires or nanotubes

with small radii or wall thicknesses are outstanding supportmaterials for fabricating photo-catalytic composites for use inH2 generation. This is because the photo-generated electronsand holes can rapidly transfer to their surfaces with minimalopportunity for recombination. It is worth nothing that con-ductive materials, such as FTO glass and titanium foil, can beused as substrates for the growth of nanoarrays. Thus photo-electrocatalytic H2 generation, which usually displays betterperformance than photo-catalysis, can be employed for nano-arrays directly grown on conductive substrates.294–297 Ourgroup fabricated TiO2 nanotube arrays sensitized with uniformand narrow-sized Pd NPs exhibiting highly efficient photo-electrocatalytic hydrogen generation.298 An improved photo-catalytic H2 production rate of 592 mmol h�1 cm�2 under320 mW cm�2 irradiation was achieved by the compositenanoarrays facilitated the charge transfer of photo-inducedelectrons from the TiO2 nanotubes to Pd NPs and the highactivity of the Pd NP photo-catalytic centers. Au NP-decoratedZnO nanorod arrays were synthesized as the photo-anode forphoto-electrochemical water splitting, as shown in Fig. 19a.136

Fig. 18 (a) H2O reduction activities of Au-P25 photo-catalysts with different Au particle sizes. (b) H2 production rates over Au/P25 TiO2 photo-catalystswith different amounts of Au loading. (c) Photo-catalytic production of H2 from ethanol over Au/TiO2 anatase and rutile as a function of Au loading. (d)Photocatalytic activity for water splitting under different light sources. Reprinted with permission from ref. 284, 282, 279 and 286. Copyright 2014American Chemical Society, 2013 Elsevier, 2011 Nature Publishing Group and 2011 American Chemical Society.


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Compared to pure ZnO nanorod arrays, the incorporation of AuNPs significantly increased the H2 generation rate and resultedin 11.2 mmol h�1 of generated H2 under solar irradiation with abias of +0.5 V. Under simulated solar irradiation, the photo-current density of the composite nanoarrays was almost twicethat of pristine ZnO nanorods indicating an improved separa-tion of electrons and holes.

Au NP-decorated ZnO nanorod/nanoplatelet compositenanoarrays were produced by a hydrothermal method.299 Thesynergy between the ZnO nanorod/nanoplatelet hierarchicalnanostructures and the plasmon resonance of Au NPs led toan increased photo-current density, enhanced incident-photon-to-current-conversion efficiency (IPCE) and improved photo-electrochemical water splitting rate. Glutathione-protected Aunanocluster-sensitized TiO2 nanotube arrays have also been usedfor photo-eletrocatalytic H2 generation under visible lightirradiation.300 During the photocatalytic process, the absorption

range of Au-decorated TiO2 nanotube arrays was extended to510 nm due to the introduction of Au NP sensitizers. Undersimulated sunlight, a maximum yield of 1.3 mmol cm�2 H2 wasobtained with the addition of Au NPs and a sacrificial agent after120 minutes of irradiation. The performance was much higherthan that of bare TiO2 nanotube arrays tested under the sameconditions.

3.3.3 Other NM–MO nanostructures. Other NM–MOnanostructures, including core/shell, yolk/shell and Janusnanostructures have also shown outstanding performance inH2 generation applications. Studies pertaining to these moreexotic morphologies are rather limited. Au/TiO2 yolk/shellnanostructures have been prepared to demonstrate the correla-tions between H2 generation rate and the PL lifetimes of thehybrid nanostructures.210 High calcination temperatures led tothe growth of grain sizes and improved TiO2 crystallinity. Thelarger particle size reduced the specific surface area which

Fig. 19 (a) Real-time photocatalytic evolution of H2 and O2 using Au/ZnO and ZnO photo-electrodes. (b) Volume of hydrogen generated and (c)Plasmonic near-field maps for different photocatalysts. Reprinted with permission from ref. 136 and 228. Copyright 2012 American Chemical Society and2012 Wiley-VCH.


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detracted from photocatalytic properties. While, H2 generationrates increased with the improvements in crystallinity due tothe lower density of defect sites. The photocatalytic activitydemonstrated that the H2 generation rate increases at higherannealing temperatures. This suggests that particle crystallinityplays a more important role in water splitting by Au/TiO2 yolk/shell composites than that of particle size. PL lifetimes areclosely tied to the degree of TiO2 crystallinity. As a consequence,the PL results can be used to estimate the potential photocata-lytic H2 generation ability of Au/TiO2 yolk/shell nanostructures.

Janus Au–TiO2 nanocomposites with 50 nm Au NPs havebeen synthesized for efficient visible light water splitting.228

The H2 generation rate of Janus Au–TiO2 was about 1.7 timeshigher than that of Au/TiO2 core/shell nanostructures. This isattributed to a reduced charge-carrier recombination due tothe shorter diffusion distance for generated electrons andenhanced energy absorption due to the stronger localizationof the plasmonic near-field in the Janus nanostructures, asshown in Fig. 19b and c. Moreover, the Au NP size also plays animportant role in the improvement of H2 generation rates ofJanus nanostructures. Specifically, Janus Au–TiO2 compositeswith large Au NPs demonstrated high performance due to thestronger plasmonic near-fields. Note that this observation iscontrary to those of previous nanocomposite systems andunderscores the significant differences associated with thephoto-catalytic mechanism of each distinct nanostructure.

3.4 Photo-catalytic CO2 reduction

Photo-catalytic CO2 reduction not only offers another strategyfor converting solar energy into chemical energy, but is also apromising approach to reduce a greenhouse gas produced byhuman industrial activities.301–304 Unlike photo-catalytic H2

generation, which only requires solar energy and water,photo-reduction of CO2 requires both energy and a hydrogensource.305–307 Suppressing the H2 generation process is anefficient strategy to improve the photo-reduction of CO2 dueto the competition between them in the presence of water.308

Since the first observation by Inoue’s group that photo-electrocatalytic reduction of CO2 over semiconductor powdersin water,309 continuous efforts have been devoted to photo-catalytic CO2 conversion.310–313 In solar energy harvesting,charge separation and transportation are two crucial factorsin the photo-catalytic CO2 reduction system. The design ofNM-decorated MOs is one of several approaches to improvethe efficiency in these two factors.

3.4.1 NM-decorated metal oxide NPs. TiO2 is frequentlyused in photo-catalytic CO2 reduction because of its abundance,stability, high UV photo-reactivity, low cost, and low toxicity.314,315

Excellent reviews have been published that focus on the photo-catalytic CO2 reduction by TiO2 and TiO2-based materials.43,307

Deposited NMs acted as co-catalysts and have been shown tobe beneficial for the efficiency of photo-catalytic CO2 reductiondue to the improved charge separation, facilitated CO2 activa-tion, and the presence of active catalytic sites for the reduc-tion process similar to photocatalytic degradation and H2

generation.43 The rate of CO2 reduction was found to correlate

with the work function of the NM decorated on the surface ofthe MOs.316 The photo-catalytic behavior of TiO2 with differentnoble metal co-catalysts was examined, and the CO2 reductionrate increased in the sequence of TiO2 o Ag–TiO2 o Rh–TiO2 oAu–TiO2 o Pd–TiO2 o Pt–TiO2. This corresponds well to theincrease in the photo-current of the different photo-catalysts. Itwas suggested that the function of NMs was to extract electronsfrom TiO2, thus promoting the performance of CO2 reduction.317

The presence of binary NMs on MOs, such as AuPt, AuAg,CuPt and AuCu, also enhanced the CO2 conversion to valuablehydrocarbons due to the synergy of two different NMs.318–324 Ithas been reported that the deposition of Au and Cu co-catalystson commercial P25 increases the efficiency of photocatalyticCO2 reduction in which the Au enabled response to visiblelight.321 Specifically, the Au–Cu co-catalysts exhibited the largestamount of CH4 production, 8–11 times larger than that obtainedwith either single-component Au or Cu-loaded P25, respectively.In addition, the ratio of the two different NMs was important tothe catalytic performance. The optimal composition was foundto be a Au/Cu ratio of 1 : 2. Granchak et al. have done someresearch on photo-catalytic CO2 conversion using TiO2 modifiedwith bimetallic Au–Cu.324 It was found that an optimal Au/Curatio existed at which the rate of photo-catalytic CH4 formationwas the highest. The size of the noble metal alloy is a key factordetermining the photo-catalytic behavior of the nanocompositecatalysts. The photo-reduction rate of CO2 improved significantlywith a decrease in the noble metal alloy size as shown in Fig. 20aand b.319 This is because the small noble metal alloy particlesstrongly bind to CO2 intermediates and have a stronger inter-action with the support.

3.4.2 NM-decorated metal oxide nanoarrays. In addition toNPs, one-dimensional (1D) metal oxide nanoarrays, such asTiO2 nanotubes and nanorods, have also been synthesizedand investigated for the photo-catalytic reduction of CO2 dueto the increased charge separation and scattering of incidentlight among nanostructures.314 1D metal oxide nanoarrayscoated with NMs have shown extremely high CO2 photo-reduction efficiency due to the synergistic combination ofthe efficient electron–hole separation by the NMs and the fastelectron-transfer rate in metal oxide nanoarrays.325 The well-ordered 1D structure of TiO2 single crystals decorated withultrafine Pt NPs synthesized via versatile gas-phase depositionmethods were used as catalysts in the photo-reduction of CO2,which showed a maximum CH4 yield of 1361 mmol g-cat�1 h�1.326

It is believed that the high surface area and single crystallinityof the 1D structure of the films and the efficient electron–holeseparation by the Pt NPs were the main reasons for theimprovement.

Bifunctional bimetallic co-catalysts are also deposited onmetal oxide nanoarrays for CO2 reduction.327–330 Cu–Pt binaryco-catalyst shells have been loaded on periodically modulateddouble-walled TiO2 nanotube arrays serving as photo-catalystswhich can capture CO2 directly from air.327 When the hetero-geneous catalyst was utilized for the photo-reduction of dilutedCO2 (0.998% in N2), an average hydrocarbon production rate of3.7 mL g�1 h�1 or 610 nmol cm�2 h�1 was achieved.


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The thickness of the nanotube wall is important to theefficiency, and a nanotube with a wall thickness less than orin the range of the minority carrier diffusion length wasbeneficial to the enhancement of the CO2 reduction rate. Highsurface area TiO2 nanotube arrays with a wall thickness lowenough to facilitate efficient transfer of photo-generated chargecarriers to the surface species have been employed as supports.Cu/Pt co-catalyst NPs were deposited on the nanotube arraysurface to adsorb the reactants and facilitate the redox process.The co-catalyst can also adjust the TiO2 band gap to utilize thevisible portion of the solar spectrum.329

Despite the rapid development of NM-decorated MOs forphotocatalytic CO2 reduction, maintaining the long-term stabi-lity and high activity of the catalyst, especially the co-catalyst, isstill a great challenge. For example, noble-metal Pt NPs areeasily poisoned by CO during the catalytic process.331 Ye et al.have fabricated Au–Cu bimetallic alloy NP-decorated SrTiO3/TiO2 coaxial nanotube arrays for high efficiency photo-catalyticconversion of CO2 into CO in hydrazine.330 Coated with anoptimized combination of Au–Cu bimetallic NPs, a CO produc-tion rate of 138.6 ppm cm�2 h�1 (3.77 mmol g�1 h�1) andtotal hydrocarbon production rate of 26.68 ppm cm�2 h�1

(725.4 mmol g�1 h�1) were achieved on SrTiO3/TiO2 coaxialnanotube arrays under UV/vis illumination due to the syner-getic catalytic effect by the Au–Cu bimetallic NPs and the fastelectron-transfer in nanotube arrays. More importantly, hydra-zine not only acted as the H source but also provided a reducingatmosphere to protect the surface Cu atoms from oxidation,maintaining the alloying effect which was the basis for the highphoto-catalytic activity and stability.

3.4.3 Other NM–MO nanostructures. Compared toNM-decorated MO structures, core (yolk)/shell and Janusstructures exhibit some special properties for optical, electricaland catalytic applications. However, only a few works on photo-catalytic CO2 reduction using these structures have beenreported. Zou et al. have fabricated Au/TiO2 yolk/shell hollowspheres as photo-catalysts for catalytic CO2 reduction rate due tothe enhanced generation of electron–hole pairs.214 In addition tothe improvement of the photo-reduction yield of CO2, chemicalreactions involving multiple e�/H+ transfer processes enable the

formation of high-grade carbon species (C2H6) due to theabundant charge-carriers generated by the electric fieldsurrounding Au NPs. Wang et al. fabricated core/shellPt/Cu2O-decorated TiO2 by a photo-deposition method usingCuSO4 and Pt/TiO2 as precursor and template respectively.318 Ithas been proposed that the Pt core extracted the photo-generated electrons from TiO2, and the Cu2O shell providedsites for the preferential activation and conversion of CO2

molecules in the presence of H2O. In addition to this, theCu2O shell on Pt markedly suppresses the H2 generationprocess, a competitive reaction with the reduction of CO2,which enabled an 85% selectivity for CO2 reduction.

3.5 Dye-sensitized solar cells

In addition to photo-catalysis, another emerging application forMOs is in photovoltaic solar cells, which can directly convertsunlight into electricity. DSSCs are one kind of third generationsolar cell possessing high efficiency and low cost. Recently, theintroduction of NMs into DSSCs to boost performance hasattracted increasing attention. Incorporating NMs enablesenhancement of incident light absorption because of the lightscattering and plasmonic effect by NMs. Also, improved chargeseparation is possible due to Schottky barrier formationbetween the NMs and MOs enabling photocurrent enhance-ment. Generally, two different approaches have been adoptedto combine NMs with DSSCs. (1) Noble metal NPs with differentsizes are first prepared and then mixed with commercial orhomemade TiO2 NPs with subsequent thin film processing on aconductive substrate.332–334 (2) Prior to fabrication into films onFTO glass, various NM–MO composites (e.g., Au NP decorated-anatase TiO2 NPs, Au NP decorated-ZnO nanotube arraysand Ag/TiO2 core/shell NPs) are fabricated as photo-anodematerials.335–339 In this section, the different nanostructuresused in DSSCs are discussed.

3.5.1 NM-decorated metal oxide NPs. Depositing plasmonicNMs on the surface of MOs is considered an effective approach toboost the efficiency of DSSCs,112,337,340–343 as shown in Fig. 21a.62

Au NP-decorated TiO2 NP composites have been synthesized andused in DSSCs.335 It is worth nothing that all the performancecharacteristics increased after the incorporation of Au NPs in

Fig. 20 (a) Production of CH4 as a function of time and (b) the production rate under a 150 W Xe lamp for CuPt–TiO2 composites of varying CuPt size.Reprinted with permission from ref. 319. Copyright 2016 Royal Society of Chemistry.


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addition to improved efficiency. Specifically, photo-conversionefficiency (PCE), current density (Jsc), open circuit voltage (Voc)and fill factor (FF) of 6%, 13.2 mA cm�2, 0.74 V and 0.61 wereobtained in TiO2–Au nanocomposite-based DSSCs. This is a clearimprovement over devices without Au NPs (PCE = 5%, Jsc =2.6 mA cm�2, Voc = 0.70 V, FF = 0.56) as shown in Fig. 21b.Similar results were also achieved in Bai’s work.340 HierarchicalTiO2 spheres decorated with Au NPs were fabricated as a novelphoto-anode material. The J–V characteristics demonstrated thatboth the PCE and Jsc increased significantly with the introductionof Au NPs. ZnO nanoflowers loaded with Au NPs have also beenproduced by a hydrothermal route and used in DSSCs.112 Theintroduction of Au NPs decreased the number of recombinationcenters generated from the oxygen vacancies in ZnO leading to asignificant enhancement in efficiency.

Various studies have shown that the loading amount andsize of NMs are key factors for determining the performance ofDSSCs. TiO2 decorated with small Au NPs (2 nm in diameter)with different contents of Au NPs have been constructed asworking electrodes for DSSCs.344 Compared to devices withbare TiO2, the efficiency increased from 5.5% to 10.1% with aJsc of 15.71 mA cm�2 and a Voc of 863 mV. More importantly, theAu loading amount was found to have a large effect on thephotovoltaic performance. When the weight ratio of Au NPs inthe composite increased from 0 to 0.800 wt%, Voc increased

while Jsc achieved a maximum value at 0.168 wt% as shown inFig. 21c. Consequently, the highest efficiency was achieved atthe ratio of 0.168%. The decrease of PCE at high Au content wasalso observed in Lim’s work.62 The negative effect was attributedto the shielding by excess Au NPs which may decrease thecontact areas between the dye and TiO2 and thus decrease theadsorption of dye molecules. TiO2 nanotubes loaded with differ-ent sizes of Au NPs were also prepared as photo-electrodes.345 Itwas found that the light response of nanocomposites can bechanged from visible to near infrared wavelengths by adjustingthe Au NPs size. Thus, composite photo-electrodes demonstratedenhanced photo-currents and IPCEs over a broad wavelengthrange from about 520–1000 nm with the incorporation ofdifferent sizes of Au NPs as shown in Fig. 21d.

3.5.2 NM-decorated metal oxide nanoarrays. The efficiencyof NP-based DSSCs is often limited by short electron diffusionlengths due to excessive defects, surface states and grainboundaries. Replacing disordered NPs with highly orderednanoarrays, such as nanorods, nanowires or nanotube arrays,is a promising solution to overcome this drawback. Comparedto NP-based DSSCs, one advantage of nanoarray-based DSSCs isthat the 1D structure provides directed electron pathways forcharge carrier transportation for improved charge collectionefficiency.346–348 While inherently low surface-to-volume ratiosremain a problem for nanoarrays,349 decoration with NMs can

Fig. 21 (a) Schematic illustration of DSSCs using TiO2/Au as a photo-anode, (b) J–V characteristics for TiO2 NPs and TiO2/Au nanocomposites, (c) J–Vcharacteristics of DSSCs based on pure TiO2, and Au–TiO2 with different amounts of Au loading, and (d) IPCE enhancement of a representative plasmon-induced photo-electrode. Reprinted with permission from ref. 62, 335, 344 and 345. Copyright 2015 Royal Society of Chemistry, 2012 Elsevier, 2013Royal Society of Chemistry and 2015 Elsevier.


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significantly enhance the dye absorbance and light absorptionin the visible wavelength.

Vertically-aligned ZnO nanorod arrays coated with Au NPshave been investigated as photo-anodes in DSSCs.146 TheSchottky barrier formed between the ZnO nanorods and Au NPsblocked the transfer of electrons from the ZnO nanorodsback to the dye and electrolyte to successfully suppress therecombination of charges. Compared to bare ZnO nanorodarrays, the efficiency of composite nanorod arrays increasedfrom 0.7% to 1.2%. Au NPs deposited on the surface of TiO2

nanorod arrays also promoted the photo-injected electrons intothe nanorod and inhibited charge carrier recombination.339

Due to modification by Au NPs, the PCE of Au NP-decoratedTiO2 nanorod array DSSCs was threefold higher than that ofdevices without Au NPs.

Nanotube arrays have a high specific surface area relative toother nanostructure arrays, which is beneficial for dye loadingand modification with NMs. ZnO nanotube arrays modifiedwith Au NPs served as a photo-anode in DSSC studies as shownin Fig. 22a.350 The combined effects of the surface plasmons ofAu NPs, large specific surface area of ZnO nanotube arrays andthe Schottky barrier at the interface of ZnO–Au compositenanoarrays led to improved DSSC performance. It was found

that Au NP sizes could be modulated by the control of theconcentration of Au precursor (i.e., AuCl3), and the highestPCE was achieved for smaller Au NPs. Ag-modified TiO2 nano-tube arrays have also been reported and showed significantenhancement in energy conversion efficiencies compared topure TiO2 nanotube arrays.351,352

3.5.3 NM/MO core (yolk)/shell nanostructures. Oneproblem associated with decorating MOs NPs or nanoarrayswith NMs is that the bare NMs can act as charge recombinationcenters and lead to corrosion by the dye/electrolytes. CoatingNMs with a MO layer to form core/shell or yolk/shell nano-structures is a promising method to solve these problems. SiO2

has been used to protect NMs from corrosion and preventrecombination.333 However, it is difficult to inject electronsinto the insulting SiO2 layer, so part of the photocurrent is lost.Ag/TiO2 core/shell nanostructures have also been investigatedfor use in DSSCs as shown in Fig. 22b.336 Due to the plasmoniceffects of the Ag NP core, optical absorption intensity andelectron collection improved significantly. Meanwhile, the out-side TiO2 shell increased the stability of the Ag NPs andreduced the loss of photo-generated electrons. The amount ofAg/TiO2 NPs in the photoanode layer (TiO2 layer) was importantto the increase in PCE. The highest PCE (9%) was achieved at a

Fig. 22 Schematic diagram of device architectures using (a) ZnO/Au composite nanoarrays, (b) Ag/TiO2 core–shell NPs as an electron conductive layer,(c) J–V characteristics of DSSCs assembled by capitalizing on three different types of photo-anodes, and (d) J–V curves obtained by (1) P25, (2) TiO2

hollow submicron spheres with thin shells and (3) Au/TiO2 yolk–shell sub-micron spheres with thin shell. Reprinted with permission from ref. 350, 336, 57and 192. Copyright 2016 Royal Society of Chemistry, 2011, American Chemical Society 2015 American Chemical Society, and 2012 Royal Society ofChemistry.


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small concentration (0.1 wt%). DSSC efficiencies were modulatedby the thickness of the TiO2 shell.353 A high Jsc was obtained withthinner shells due to the lower electronic resistance. An enhancedVoc was achieved with thicker shells due to the modified Fermilevel of TiO2. Au/TiO2/PS core–shell–shell NPs have been preparedthat show outstanding photovoltaic performance.57 Both the bareAu/TiO2 NPs and carbon-coated Au/TiO2 NPs could be obtainedat different annealing atmospheres. Compared to pure TiO2,the efficiency of Au/TiO2-based DSSCs increased by 7.4%.Carbon-coated Au/TiO2 NPs demonstrated improved electronconductivity in the outer layer leading to further improvementin PCE (a 13.6% increase) as shown in Fig. 22c.

Au/TiO2 yolk/shell nanostructures with controllable sizesand shell thicknesses have been used as photo-anodes inDSSCs.192 To the best of our knowledge, this is the only workthat has investigated the function of yolk/shell nanostructuresin DSSCs. It was found that yolk/shell nanostructures with thinshells had better performance than those with thicker shellsdue to the reduction of photo-generated electron–hole recom-bination. Compared to pure TiO2 electrodes, with a PCE of6.25%, the best efficiency of yolk–shell-based DSSCs with thinshells was 8.13% and exhibited a 30% enhancement as shownin Fig. 22d.

3.6 Perovskite solar cells (PSCs)

A successor to DSSCs, PSCs are promising photovoltaic devicesdue to the unprecedented rise in the PCE performance and lowdevice cost.354,355 From the first report about all-solid-state PSCwith a PCE of 9.7% in 2012, the highest PCE to date (20.1%) wasachieved in late 2014.356,357 Recently, NM–MO hybrid nano-structures have been used in PSCs to increase the efficiency.The light absorption can be enhanced through the incorpora-tion of NMs which provide effective light absorption due tosurface plasmon excitation and extended optical path-lengthfrom light scattering.328 However bare NMs can act as recom-bination centers for the photo-generated charges and may reactwith the iodide in the perovskite layer. Thus a covering layerwith a dielectric shell is essential for the protection of the NMs.Snaith et al. have fabricated Ag/TiO2 core/shell nanostructuresthat were then incorporated into PSCs.358 The shell thickness

and the concentration of Ag/TiO2 in the mesoporous layer playan important role in the modification of the PCE. With theintroduction of optimized Ag/TiO2 NPs, the PCE increased byabout 20% compared to the control devices (Fig. 23a). Theimprovement in PCE is attributed primarily to the enhance-ment of Jsc since the Voc and fill factor remained virtuallyunchanged after the incorporation of Ag/TiO2. The enhancedJsc was also observed in Au decorated TiO2 nanofiber-basedPSCs (Fig. 23b).359 It was noted that the stability of TiO2/Aunanofiber-based PSCs was much higher than that of pure TiO2

fiber-based PSCs because of the lower degree of degradation ofMAPbI3 perovskites.

4. Conclusions and outlook

In recent years, the extensive research of noble metal–metaloxide nanocomposites has demonstrated their key role in theconversion of solar energy into electrical and chemical energy.The structure of the composites is critical in determining theirphoto-catalytic and photovoltaic performance. The hybridnanostructures of noble metal–metal oxides can be generallydivided into (a) NM-decorated metal oxide NPs, (b) NM-decoratedmetal oxide nanoarrays, (c) noble metal/metal oxide core/shellnanostructures, (d) noble metal/metal oxide yolk/shell nanostruc-tures, and (e) Janus noble metal–metal oxide nanostructures. In thisreview, we have summarized the properties, preparation methodsand applications of noble metal–metal oxide nanocomposites andrationalized the observed behaviors for each structure. Generally, itis easy to deposit noble metals on the surface of metal oxides. Incomparison, the preparation of other nanostructures, core/shell,yolk/shell and Janus, is relatively complicated. The photocatalyticdegradation, photocatalytic H2 generation and photovoltaic efficien-cies of metal oxides are all greatly enhanced due to the plasmonresonance of noble metals and the formation of Schottky barriers atthe interfaces of the metal oxide and noble metal. Rapid progress inthe development of noble metal–metal oxide nanocomposites hasbeen made. However, many challenges remain to be addressed.

For noble metal-decorated metal oxide NPs, the loadingamount and uniformity of the noble metals are extremely

Fig. 23 (a) J–V characteristics for the best control device and optimized Ag/TiO2 device (b) FE-SEM cross-sectional image of a perovskite solar cellbased on Au decorated TiO2 nanofibers. Reprinted with permission from ref. 358 and 359. Copyright 2015 Wiley-VCH and 2016 Royal Society ofChemistry.


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important for dictating the properties of the resulting nano-structures. However, it is difficult to precisely modulate thesekey parameters using current methods. Thus, new preparativeroutes or modifications of existing synthetic methods arenecessary in this area. An alternative templating method tosynthesize core/shell nanoparticles using amphiphilic star-likeP4VP-b-PAA-b-PS or all hydrophilic star-like P4VP-b-PAA-b-PEOtriblock copolymer nanoreactors is also a promising option. Inthis method, the core diameter as well as the shell thicknesscan be precisely controlled by tailoring the length of the P4VPor PAA chains respectively.187 Subsequent deposition of NMson the surface of MOs with accurate loading amounts andsuperior uniformity can be achieved. The growth of the MO(and NM) part is based on the coordination between P4VP(and PAA) blocks in the star-like triblock copolymer and the NMprecursors.

Research into noble metal/metal oxide core/shell or yolk/shell structures focuses mainly on NPs as the core material.While other shapes of noble metal cores, such as 1D nanorodsand 2D nanosheets, are rarely reported. The properties ofnanomaterials are closely related to their morphologies. Thusthe applications of core/shell or yolk/shell composites can bevaried by coating various kinds of noble metal cores. For example,an obvious absorption peak in the infrared region was demon-strated in Au nanorod/TiO2 yolk/shell nanostructures.207 UsingAu/Ag alloy nanorods as core materials, the photo-catalytic effi-ciency of Au/Ag/TiO2 core/shell NPs is significantly improvedwhen compared to Au nanorod/TiO2.360 In order to synthesizecore/shell or yolk/shell nanostructures with 1D or 2D NMs as thecore, some novel synthetic strategies are required. For example,using amphiphilic bottlebrush-like P4VP-b-PAA-b-PS or all hydro-philic bottlebrush-like P4VP-b-PAA-b-PEO triblock copolymers ascylindrical templates, Au nanorod/TiO2 core/shell nanostructurescan be produced based on strong coordination bonding betweenthe Au (and TiO2) precursors and P4VP (and PAA) blocks. Addi-tionally, intriguing nanoparticles with complex core/shell struc-tures, e.g., noble metal core/yolk/metal oxide shells or onion-likenoble metal core/metal oxide shell 1/metal oxide shell 2, may becrafted using star-like tetrablock copolymers as nanoreactorswith the possibility to integrate multi-functionality into a singlestructure.

Although Janus structures were first proposed in 1991,research into Janus noble metal–metal oxide nanostructures,especially for TiO2 and ZnO, is still in its infancy. Robustpreparation strategies are still needed to construct well-defined Janus noble metal–metal oxide nanostructures withcontrollable size and content. The applications of Janus nano-structures are limited to photo-catalysis while no work aboutthe photovoltaic properties has been reported. Thus, otherfunctionalities of Janus nanostructures need to be explored.For example, Janus Au–TiO2 can be deposited at the interfacebetween the compact TiO2 layer and the perovskite layer ofplanar perovskite solar cells (PSCs) with the TiO2 side facing upand Au side facing down by the surface modification of Janusparticles. The upward-facing TiO2 prevents the direct contactbetween the Au and perovskite layers, and the downward-facing

Au acts as a conductor to greatly improve the charge extractionfrom the perovskite layer. In addition, the absorption intensityof PSCs can be increased due to the scattering and plasmoniceffects of Au. As a consequence, the performance of PSCs can beimproved via the introduction of Janus Au–TiO2.

Since infrared light is a large part of the solar spectrum,solar energy-related performance can be effectively improvedby harnessing infrared light. For example, upconversionNaYF4:Yb/Er NPs have recently been investigated to harvestthe near-infrared solar photons and serve as a mesoporouslayer for PSCs.361 The PCE of upconversion NPs-based PSCs hasreached as high as 18.1% due to a harvesting of infraredphotons. The absorption spectra of Au nanorods can be tunedfrom visible to near infrared through the modulation of theaspect ratio of the Au nanorods. Therefore, the combination ofmetal oxide and Au nanorods is an alternative means ofcapturing infrared light for use in solar energy conversion.Specifically, PSCs can absorb UV to near infrared (200–800 nm)light. On the other hand, the absorption band along the trans-verse direction and longitudinal direction of Au nanorods can beprecisely controlled in the range of 500–550 nm and 800–900 nmvia the adjustment of size or aspect ratio, respectively. Thus, theabsorption intensity of PSC in visible region can be strengthenedand the absorption range can be extended to 900 nm byincorporating Au nanorods into PSCs. The generation of elec-trons and holes under low energy irradiation has already beendemonstrated in Au nanorods/TiO2 systems.362 The excitedelectrons from Au nanorods can be quickly transferred to theconduction band of TiO2 due to the direct interband excitation.This increases the photo-current, and improves the PCE inperovskite solar cells.

Noble metal–metal oxide nanocomposites can be used totransform solar light into electrical energy for subsequent usein water splitting. The electricity transformed from solar energyneeds to be stored with hydrogen generation as one process forenergy storage. As a result, the integration of these separateapplications into one device can be a highly desirable andpromising research area. For example, recently a high solar-to-hydrogen efficiency for water splitting has been demonstratedby the combination of a BiVO4 photo-anode and a PSC.363 Inanother account, a Pt/CdS photo-catalyst was introduced into alithium–sulfur battery to realize a solar-driven chargeablelithium-ion battery enabling the direct conversion and storageof abundant but intermittent solar energy.364 In anotheraccount, graphene-decorated TiO2 nanorod arrays were usedas the photo-anode of a fuel cell enabling the synergisticconversion of chemical and solar energy.365 It is worth notingthat more complex systems involving three or more types ofnanocomposites may also be beneficial for solar energy con-version, such as in the integration of solar cells, photocatalyticmaterials and lithium-ion batteries into one device.

As summarized in this review, noble metal–metal oxidenanocomposites with different nanostructures have showngreat potential for solar energy conversion. Clearly, efforts in theexploration of the abovementioned challenges will lead to manybreakthroughs in traditional photo-catalytic and photovoltaic


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nanomaterials as well as further work to promote the use of solarenergy in our daily lives.

Acknowledgements

This work was supported by the Fund for Outstanding DoctoralDissertations of the China University of Geosciences, theChinese Scholarship Council, and the Air Force Office ofScientific Research (FA9550-16-1-0187).

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